UI redesign Phase 1: NavigationSplitView with collapsible sidebar

- Replace root VStack with NavigationSplitView (2-column, collapsible sidebar)
- Add SidebarView: new chat button, conversation search, list with swipe actions
- Slim HeaderView to text-only (provider + model + star); remove all icon rows
- Move status pills (Online, MCP, Synced) to footer right side
- Remove version number and shortcut hints from footer
- Add resizable InputBar with drag handle (persisted height) and globe/network.slash online toggle
- Fix Norwegian menu appearing on English systems (CFBundleLocalizations in Info.plist)
- Add View menu (Model Info, History, Stats, Credits, Online Mode toggle ⌘⇧O)
- Add ⌘L as alias for Search Conversations (muscle memory for /load users)
- Add Check for Updates to Help menu with download URL from Gitea API
- Add one-time Intel/Rosetta deprecation warning on first launch
- Swift 6: fix self.Self.isoString() call sites in DatabaseService

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

oAI/Services/GitSyncService.swift

@@ -212,7 +212,7 @@ class GitSyncService {
 
                 // Check if conversation already exists (by ID)
                 if let existingId = UUID(uuidString: export.id) {
-                    if let existing = try? db.loadConversation(id: existingId) {
+                    if (try? db.loadConversation(id: existingId)) != nil {
                         // Already exists - skip
                         log.debug("Skipping existing conversation: \(export.name)")
                         skipped += 1