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/ui/src/lib/settings/search.ts

@@ -370,6 +370,14 @@ const SETTINGS_SEARCH_ITEMS: readonly SettingsSearchItem[] = [
     keywords: ['desktop', 'startup', 'login'],
     isAvailable: (ctx) => ctx.isDesktopLocalOrigin,
   },
+  {
+    id: 'sessions.desktop-window-controls-position',
+    page: 'sessions',
+    titleKey: 'settings.openchamber.desktopNetwork.field.windowControlsPosition',
+    descriptionKey: 'settings.openchamber.desktopNetwork.field.windowControlsPositionDescription',
+    keywords: ['desktop', 'window', 'controls', 'minimize', 'maximize', 'close', 'titlebar', 'linux', 'windows'],
+    isAvailable: (ctx) => ctx.isDesktop && (ctx.isWindows || !ctx.isMac),
+  },
   {
     id: 'sessions.desktop-minimize-to-tray',
     page: 'sessions',