M19: Move sidebarCollapsed from localStorage to electron-store

Sidebar collapsed state was an orphan in localStorage -- not backed up,
lost on portable installs, and inconsistent with every other preference.

Added sidebarCollapsed: boolean to the Settings type and DEFAULT_SETTINGS,
wired it through applySettings' boolean branch, and updated the renderer
store / main.tsx mock defaults. App.tsx now drives the toggle via
updateSettings({ sidebarCollapsed }) instead of localStorage.

typecheck + 242 tests + eslint green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/settings.ts

@@ -64,6 +64,7 @@ const DEFAULTS: Settings = {
   hasCompletedOnboarding: false,
   minimizeToTray: false,
   launchAtStartup: false,
+  sidebarCollapsed: false,
   updateToken: ''
 }
 
@@ -335,6 +336,7 @@ function applySettings(s: Store<Settings>, partial: Partial<Settings>): void {
       case 'autoDownloadNew':
       case 'hasCompletedOnboarding':
       case 'minimizeToTray':
+      case 'sidebarCollapsed':
         if (typeof value === 'boolean') s.set(key, value)
         break
       case 'launchAtStartup':