feat(desktop): Linux AppImage polish — window controls, updater UX, docs (#2144)

* feat(electron): add Linux AppImage releases

* ci: cache Linux OpenCode CLI artifacts

* fix(ci): await Linux release inventory check

* fix(electron): add frameless window controls on Linux desktop

Linux AppImages were created without native WM decorations and without
in-app controls, leaving users unable to close the window with a mouse.

Treat Linux like Windows: frameless BrowserWindow plus the existing
WindowsWindowControls header buttons and app-menu entry. macOS keeps
hidden title bar with traffic lights unchanged.

Shared usesFramelessElectronChrome() helper drives main window, mini
chat, header insets, and titlebar controls.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

* feat(desktop): add configurable window controls position by OS

Add desktopWindowControlsPosition setting (auto/left/right) with OS-aware
defaults: Linux left, Windows right. Wire frameless chrome controls in
Header, TitlebarLeftControls, and MiniChatLayout, plus a Sessions settings
control for Windows and Linux desktop shells.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

* fix(desktop): address Linux AppImage release review findings

Propagate updater capability errors to the UI, treat missing
latest-linux.yml feeds as no-update, stop installed-apps IPC spam on
Linux, document FUSE/AppImage limits, add CHANGELOG entry, migrate
remaining btriapitsyn URLs, and run Electron Linux unit tests on PRs.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

---------

Co-authored-by: jibanez-staticduo <staticduo@gmail.com>
Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/electron/package.json

@@ -21,7 +21,8 @@
     "Electron runtime dependencies installed via bun install",
     "Bun available for sidecar compilation",
     "macOS: Xcode + build tools for notarized packaging",
-    "Windows: NSIS installed for installer creation"
+    "Windows: NSIS installed for installer creation",
+    "Linux: native x64 or arm64 host (no cross-arch packaging); FUSE/libfuse2 to run AppImages, or APPIMAGE_EXTRACT_AND_RUN=1"
   ],
   "scripts": {
     "dev": "node ./scripts/electron-dev.mjs",
@@ -30,9 +31,14 @@
     "prepare:opencode-cli": "node ./scripts/prepare-opencode-cli.mjs",
     "verify:opencode-cli": "node ./scripts/verify-opencode-cli.mjs --staged",
     "verify:opencode-cli:packaged": "node ./scripts/verify-opencode-cli.mjs --packaged",
+    "verify:linux-appimage": "node ./scripts/verify-linux-appimage.mjs",
     "bundle:main": "bun ./scripts/bundle-main.mjs",
     "generate:macos-icon": "node ./scripts/generate-macos-icon-assets.cjs",
     "rebuild:native": "node ./scripts/rebuild-native.mjs",
+    "test:architecture": "node --test ./scripts/target-architecture.test.mjs ./scripts/verify-linux-appimage.test.mjs ./scripts/verify-update-manifest.test.mjs",
+    "test:updater": "node --test ./updater-capability.test.mjs ./updater-check.test.mjs ./updater-feed.test.mjs ./scripts/updater-e2e-fixture.test.mjs",
+    "updater:e2e:fixture": "node ./scripts/updater-e2e-fixture.mjs",
+    "verify:update-manifest": "node ./scripts/verify-update-manifest.mjs",
     "package": "bun run build:web-assets && bun run prepare:opencode-cli && bun run bundle:main && bun run rebuild:native && node ./scripts/package.mjs",
     "finalize:latest-yml": "node ./scripts/finalize-latest-yml.mjs",
     "type-check": "node --check ./main.mjs && node --check ./preload.mjs",
@@ -54,6 +60,10 @@
         "from": "resources/icons/icon.ico",
         "to": "icons/icon.ico"
       },
+      {
+        "from": "resources/icons/icon.png",
+        "to": "icons/icon.png"
+      },
       {
         "from": "resources/icons/tray",
         "to": "icons/tray"
@@ -95,6 +105,23 @@
       "verifyUpdateCodeSignature": false,
       "artifactName": "${productName}-${version}-win-${arch}.${ext}"
     },
+    "linux": {
+      "target": [
+        "AppImage"
+      ],
+      "category": "Development",
+      "icon": "resources/icons/icon.png",
+      "executableName": "openchamber",
+      "artifactName": "${productName}-${version}-linux-${arch}.${ext}",
+      "desktop": {
+        "entry": {
+          "Name": "OpenChamber",
+          "Comment": "Desktop runtime for OpenChamber",
+          "Icon": "openchamber",
+          "StartupWMClass": "openchamber"
+        }
+      }
+    },
     "nsis": {
       "oneClick": true,
       "perMachine": false,