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/stores/useUIStore.ts

@@ -20,6 +20,7 @@ export type ActivityRenderMode = 'collapsed' | 'summary';
 export type SessionRetentionAction = 'archive' | 'delete';
 export type TimeFormatPreference = 'auto' | '12h' | '24h';
 export type WeekStartPreference = 'auto' | 'sunday' | 'monday';
+export type DesktopWindowControlsPosition = 'auto' | 'left' | 'right';
 export type FileEditorKeymap = 'default' | 'vim';
 
 function normalizeFileEditorKeymap(value: unknown): FileEditorKeymap {
@@ -649,6 +650,7 @@ interface UIStore {
   showExpandedEditTools: boolean;
   timeFormatPreference: TimeFormatPreference;
   weekStartPreference: WeekStartPreference;
+  desktopWindowControlsPosition: DesktopWindowControlsPosition;
   mermaidRenderingMode: MermaidRenderingMode;
   userMessageRenderingMode: UserMessageRenderingMode;
   collapsibleUserMessages: boolean;
@@ -803,6 +805,7 @@ interface UIStore {
   setShowExpandedEditTools: (value: boolean) => void;
   setTimeFormatPreference: (value: TimeFormatPreference) => void;
   setWeekStartPreference: (value: WeekStartPreference) => void;
+  setDesktopWindowControlsPosition: (value: DesktopWindowControlsPosition) => void;
   setMermaidRenderingMode: (value: MermaidRenderingMode) => void;
   setUserMessageRenderingMode: (value: UserMessageRenderingMode) => void;
   setCollapsibleUserMessages: (value: boolean) => void;
@@ -950,6 +953,7 @@ export const useUIStore = create<UIStore>()(
         showExpandedEditTools: false,
         timeFormatPreference: 'auto',
         weekStartPreference: 'auto',
+        desktopWindowControlsPosition: 'auto',
         mermaidRenderingMode: 'svg',
         userMessageRenderingMode: 'markdown',
         collapsibleUserMessages: true,
@@ -2100,6 +2104,9 @@ export const useUIStore = create<UIStore>()(
         setWeekStartPreference: (value) => {
           set({ weekStartPreference: value });
         },
+        setDesktopWindowControlsPosition: (value) => {
+          set({ desktopWindowControlsPosition: value });
+        },
         setMermaidRenderingMode: (value) => {
           set({ mermaidRenderingMode: value });
         },
@@ -2357,6 +2364,7 @@ export const useUIStore = create<UIStore>()(
           showExpandedEditTools: state.showExpandedEditTools,
           timeFormatPreference: state.timeFormatPreference,
           weekStartPreference: state.weekStartPreference,
+          desktopWindowControlsPosition: state.desktopWindowControlsPosition,
           mermaidRenderingMode: state.mermaidRenderingMode,
           userMessageRenderingMode: state.userMessageRenderingMode,
           collapsibleUserMessages: state.collapsibleUserMessages,