feat(desktop): inline @-mentions as ProseMirror pills in the composer

The composer was a plain textarea, so @-selected files and folders could
not render inline: they became attachment chips above the input and were
sent as a detached @"path" prefix. The composer now runs on ProseMirror
(ChatGPT desktop's architecture): @ selections insert an inline mention
pill (an atom node carrying label/path/isDirectory), and the pill is
serialized back to the @"absolute path" text the CLI already parses at
its exact position in the prompt.

- MentionComposer + composerEditor: schema, doc<->projection mapping,
  controlled bridge (doc changes project to text+mentions; external
  writes rebuild the editor state, resetting undo history), single-
  backspace whole-pill deletion, placeholder/disabled/aria plumbing
- composerMentions: tokenOrdinal keeps pills distinct from literal text
  that happens to spell the same token, so serialization and rebuilds
  never rewrite the wrong occurrence
- ChatInput and EmptySession composers both migrated; drafts persist
  mentions; drag-and-drop attachment chips unchanged

desktop/bun.lock

@@ -21,6 +21,12 @@
         "mermaid": "^11.14.0",
         "node-pty": "^1.1.0",
         "prism-react-renderer": "^2.4.1",
+        "prosemirror-commands": "^1.7.1",
+        "prosemirror-history": "^1.5.0",
+        "prosemirror-keymap": "^1.2.3",
+        "prosemirror-model": "^1.25.11",
+        "prosemirror-state": "^1.4.4",
+        "prosemirror-view": "^1.42.2",
         "qrcode": "^1.5.4",
         "react": "^18.3.1",
         "react-diff-viewer-continued": "^4.2.0",
@@ -1202,6 +1208,8 @@
 
     "oniguruma-to-es": ["oniguruma-to-es@4.3.6", "https://registry.npmmirror.com/oniguruma-to-es/-/oniguruma-to-es-4.3.6.tgz", { "dependencies": { "oniguruma-parser": "^0.12.2", "regex": "^6.1.0", "regex-recursion": "^6.0.2" } }, "sha512-csuQ9x3Yr0cEIs/Zgx/OEt9iBw9vqIunAPQkx19R/fiMq2oGVTgcMqO/V3Ybqefr1TBvosI6jU539ksaBULJyA=="],
 
+    "orderedmap": ["orderedmap@2.1.1", "https://registry.npmmirror.com/orderedmap/-/orderedmap-2.1.1.tgz", {}, "sha512-TvAWxi0nDe1j/rtMcWcIj94+Ffe6n7zhow33h40SKxmsmozs6dz/e+EajymfoFcHd7sxNn8yHM8839uixMOV6g=="],
+
     "p-cancelable": ["p-cancelable@2.1.1", "https://registry.npmmirror.com/p-cancelable/-/p-cancelable-2.1.1.tgz", {}, "sha512-BZOr3nRQHOntUjTrH8+Lh54smKHoHyur8We1V8DSMVrl5A2malOOwuJRnKRDjSnkoeBh4at6BwEnb5I7Jl31wg=="],
 
     "p-limit": ["p-limit@3.1.0", "https://registry.npmmirror.com/p-limit/-/p-limit-3.1.0.tgz", { "dependencies": { "yocto-queue": "^0.1.0" } }, "sha512-TYOanM3wGwNGsZN2cVTYPArw454xnXj5qmWF1bEoAc4+cU/ol7GVh7odevjp1FNHduHc3KZMcFduxU5Xc6uJRQ=="],
@@ -1276,6 +1284,20 @@
 
     "property-information": ["property-information@7.1.0", "https://registry.npmmirror.com/property-information/-/property-information-7.1.0.tgz", {}, "sha512-TwEZ+X+yCJmYfL7TPUOcvBZ4QfoT5YenQiJuX//0th53DE6w0xxLEtfK3iyryQFddXuvkIk51EEgrJQ0WJkOmQ=="],
 
+    "prosemirror-commands": ["prosemirror-commands@1.7.1", "https://registry.npmmirror.com/prosemirror-commands/-/prosemirror-commands-1.7.1.tgz", { "dependencies": { "prosemirror-model": "^1.0.0", "prosemirror-state": "^1.0.0", "prosemirror-transform": "^1.10.2" } }, "sha512-rT7qZnQtx5c0/y/KlYaGvtG411S97UaL6gdp6RIZ23DLHanMYLyfGBV5DtSnZdthQql7W+lEVbpSfwtO8T+L2w=="],
+
+    "prosemirror-history": ["prosemirror-history@1.5.0", "https://registry.npmmirror.com/prosemirror-history/-/prosemirror-history-1.5.0.tgz", { "dependencies": { "prosemirror-state": "^1.2.2", "prosemirror-transform": "^1.0.0", "prosemirror-view": "^1.31.0", "rope-sequence": "^1.3.0" } }, "sha512-zlzTiH01eKA55UAf1MEjtssJeHnGxO0j4K4Dpx+gnmX9n+SHNlDqI2oO1Kv1iPN5B1dm5fsljCfqKF9nFL6HRg=="],
+
+    "prosemirror-keymap": ["prosemirror-keymap@1.2.3", "https://registry.npmmirror.com/prosemirror-keymap/-/prosemirror-keymap-1.2.3.tgz", { "dependencies": { "prosemirror-state": "^1.0.0", "w3c-keyname": "^2.2.0" } }, "sha512-4HucRlpiLd1IPQQXNqeo81BGtkY8Ai5smHhKW9jjPKRc2wQIxksg7Hl1tTI2IfT2B/LgX6bfYvXxEpJl7aKYKw=="],
+
+    "prosemirror-model": ["prosemirror-model@1.25.11", "https://registry.npmmirror.com/prosemirror-model/-/prosemirror-model-1.25.11.tgz", { "dependencies": { "orderedmap": "^2.0.0" } }, "sha512-QWg9RhnpLlogAmp3p96uEFrE5txQpFynd4vhBAELkwgOCWQs/X0yCzB3/hrHqiPwf91RG5KyWq6553zs9JqIOQ=="],
+
+    "prosemirror-state": ["prosemirror-state@1.4.4", "https://registry.npmmirror.com/prosemirror-state/-/prosemirror-state-1.4.4.tgz", { "dependencies": { "prosemirror-model": "^1.0.0", "prosemirror-transform": "^1.0.0", "prosemirror-view": "^1.27.0" } }, "sha512-6jiYHH2CIGbCfnxdHbXZ12gySFY/fz/ulZE333G6bPqIZ4F+TXo9ifiR86nAHpWnfoNjOb3o5ESi7J8Uz1jXHw=="],
+
+    "prosemirror-transform": ["prosemirror-transform@1.12.0", "https://registry.npmmirror.com/prosemirror-transform/-/prosemirror-transform-1.12.0.tgz", { "dependencies": { "prosemirror-model": "^1.21.0" } }, "sha512-GxboyN4AMIsoHNtz5uf2r2Ru551i5hWeCMD6E2Ib4Eogqoub0NflniaBPVQ4MrGE5yZ8JV9tUHg9qcZTTrcN4w=="],
+
+    "prosemirror-view": ["prosemirror-view@1.42.2", "https://registry.npmmirror.com/prosemirror-view/-/prosemirror-view-1.42.2.tgz", { "dependencies": { "prosemirror-model": "^1.25.8", "prosemirror-state": "^1.0.0", "prosemirror-transform": "^1.1.0" } }, "sha512-Pdg0l5kXm8aLDquFAnQFTCITg0q44sLqBlHlpsVLD9segdOao8TOfQdAhCrCXyVgPSRr6UDDROOIWA3bIrN9YQ=="],
+
     "pump": ["pump@3.0.4", "https://registry.npmmirror.com/pump/-/pump-3.0.4.tgz", { "dependencies": { "end-of-stream": "^1.1.0", "once": "^1.3.1" } }, "sha512-VS7sjc6KR7e1ukRFhQSY5LM2uBWAUPiOPa/A3mkKmiMwSmRFUITt0xuj+/lesgnCv+dPIEYlkzrcyXgquIHMcA=="],
 
     "punycode": ["punycode@2.3.1", "https://registry.npmmirror.com/punycode/-/punycode-2.3.1.tgz", {}, "sha512-vYt7UD1U9Wg6138shLtLOvdAu+8DsC/ilFtEVHcH+wydcSpNE20AfSOduf6MkRFahL5FY7X1oU7nKVZFtfq8Fg=="],
@@ -1330,6 +1352,8 @@
 
     "rollup": ["rollup@4.60.1", "https://registry.npmmirror.com/rollup/-/rollup-4.60.1.tgz", { "dependencies": { "@types/estree": "1.0.8" }, "optionalDependencies": { "@rollup/rollup-android-arm-eabi": "4.60.1", "@rollup/rollup-android-arm64": "4.60.1", "@rollup/rollup-darwin-arm64": "4.60.1", "@rollup/rollup-darwin-x64": "4.60.1", "@rollup/rollup-freebsd-arm64": "4.60.1", "@rollup/rollup-freebsd-x64": "4.60.1", "@rollup/rollup-linux-arm-gnueabihf": "4.60.1", "@rollup/rollup-linux-arm-musleabihf": "4.60.1", "@rollup/rollup-linux-arm64-gnu": "4.60.1", "@rollup/rollup-linux-arm64-musl": "4.60.1", "@rollup/rollup-linux-loong64-gnu": "4.60.1", "@rollup/rollup-linux-loong64-musl": "4.60.1", "@rollup/rollup-linux-ppc64-gnu": "4.60.1", "@rollup/rollup-linux-ppc64-musl": "4.60.1", "@rollup/rollup-linux-riscv64-gnu": "4.60.1", "@rollup/rollup-linux-riscv64-musl": "4.60.1", "@rollup/rollup-linux-s390x-gnu": "4.60.1", "@rollup/rollup-linux-x64-gnu": "4.60.1", "@rollup/rollup-linux-x64-musl": "4.60.1", "@rollup/rollup-openbsd-x64": "4.60.1", "@rollup/rollup-openharmony-arm64": "4.60.1", "@rollup/rollup-win32-arm64-msvc": "4.60.1", "@rollup/rollup-win32-ia32-msvc": "4.60.1", "@rollup/rollup-win32-x64-gnu": "4.60.1", "@rollup/rollup-win32-x64-msvc": "4.60.1", "fsevents": "~2.3.2" }, "bin": "dist/bin/rollup" }, "sha512-VmtB2rFU/GroZ4oL8+ZqXgSA38O6GR8KSIvWmEFv63pQ0G6KaBH9s07PO8XTXP4vI+3UJUEypOfjkGfmSBBR0w=="],
 
+    "rope-sequence": ["rope-sequence@1.3.4", "https://registry.npmmirror.com/rope-sequence/-/rope-sequence-1.3.4.tgz", {}, "sha512-UT5EDe2cu2E/6O4igUr5PSFs23nvvukicWHx6GnOPlHAiiYbzNuCRQCuiUdHJQcqKalLKlrYJnjY0ySGsXNQXQ=="],
+
     "roughjs": ["roughjs@4.6.6", "https://registry.npmmirror.com/roughjs/-/roughjs-4.6.6.tgz", { "dependencies": { "hachure-fill": "^0.5.2", "path-data-parser": "^0.1.0", "points-on-curve": "^0.2.0", "points-on-path": "^0.2.1" } }, "sha512-ZUz/69+SYpFN/g/lUlo2FXcIjRkSu3nDarreVdGGndHEBJ6cXPdKguS8JGxwj5HA5xIbVKSmLgr5b3AWxtRfvQ=="],
 
     "rrweb-cssom": ["rrweb-cssom@0.7.1", "https://registry.npmmirror.com/rrweb-cssom/-/rrweb-cssom-0.7.1.tgz", {}, "sha512-TrEMa7JGdVm0UThDJSx7ddw5nVm3UJS9o9CCIZ72B1vSyEZoziDqBYP3XIoi/12lKrJR8rE3jeFHMok2F/Mnsg=="],
@@ -1520,6 +1544,8 @@
 
     "vscode-uri": ["vscode-uri@3.1.0", "https://registry.npmmirror.com/vscode-uri/-/vscode-uri-3.1.0.tgz", {}, "sha512-/BpdSx+yCQGnCvecbyXdxHDkuk55/G3xwnC0GqY4gmQ3j+A+g8kzzgB4Nk/SINjqn6+waqw3EgbVF2QKExkRxQ=="],
 
+    "w3c-keyname": ["w3c-keyname@2.2.8", "https://registry.npmmirror.com/w3c-keyname/-/w3c-keyname-2.2.8.tgz", {}, "sha512-dpojBhNsCNN7T82Tm7k26A6G9ML3NkhDsnw9n/eoxSRlVBB4CEtIQ/KTCLI2Fwf3ataSXRhYFkQi3SlnFwPvPQ=="],
+
     "w3c-xmlserializer": ["w3c-xmlserializer@5.0.0", "https://registry.npmmirror.com/w3c-xmlserializer/-/w3c-xmlserializer-5.0.0.tgz", { "dependencies": { "xml-name-validator": "^5.0.0" } }, "sha512-o8qghlI8NZHU1lLPrpi2+Uq7abh4GGPpYANlalzWxyWteJOCsr/P+oPBA49TOLu5FTZO4d3F9MnWJfiMo4BkmA=="],
 
     "webidl-conversions": ["webidl-conversions@7.0.0", "https://registry.npmmirror.com/webidl-conversions/-/webidl-conversions-7.0.0.tgz", {}, "sha512-VwddBukDzu71offAQR975unBIGqfKZpM+8ZX6ySk8nYhVoo5CYaZyzt3YBvYtRtO+aoGlqxPg/B87NGVZ/fu6g=="],