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/hooks/useDesktopWindowControlsLayout.ts

@@ -0,0 +1,21 @@
+import { useMemo } from 'react';
+
+import {
+  resolveDesktopWindowControlsSide,
+  usesFramelessElectronChrome,
+  type DesktopWindowControlsSide,
+} from '@/lib/desktop';
+import { useUIStore } from '@/stores/useUIStore';
+
+export function useDesktopWindowControlsLayout(): {
+  usesFramelessChrome: boolean;
+  side: DesktopWindowControlsSide;
+} {
+  const preference = useUIStore((state) => state.desktopWindowControlsPosition);
+
+  return useMemo(() => {
+    const usesFramelessChrome = usesFramelessElectronChrome();
+    const side = resolveDesktopWindowControlsSide(preference);
+    return { usesFramelessChrome, side };
+  }, [preference]);
+}