refactor(ios): restructure app with TabView at root

[datlechin]

Summary

Major architecture refactor: replaces NavigationSplitView + segmented control with TabView at the app root after connecting. This is the correct Apple pattern for tab-based navigation with NavigationStack per tab.

Before

NavigationSplitView
  ├── sidebar: ConnectionListView
  └── detail: NavigationStack → ConnectedView → segmented [Tables|Query]

After

[no activeConnection] → NavigationStack { ConnectionListView (full screen) }
[activeConnection]    → TabView (root)
                          ├── Tab "Tables": NavigationStack { TableListView → ... }
                          └── Tab "Query": NavigationStack { QueryEditorView → ... }

Changes

• AppState: Added activeConnection: DatabaseConnection? to control root transition
• ConnectedTabView (new): TabView with NavigationStack per tab, replaces ConnectedView
• ConnectionListView: Removed NavigationSplitView, standalone list with Button taps
• TableProMobileApp: New root conditional, deep-link/Handoff handlers set activeConnection
• ConnectedView: Deleted (replaced by ConnectedTabView)

Benefits

• Each tab owns its own NavigationStack — toolbar items propagate correctly
• TabView preserves both tab contents in memory (query text survives tab switches)
• No nested NavigationStack issues
• "< Connections" button in toolbar to disconnect and return to connection list
• Clean separation: connection list is the launch screen, TabView is the connected experience

Test plan

• Launch → full-screen ConnectionListView with large title "Connections"
• Tap connection → TabView with bottom tab bar (Tables/Query)
• Tables tab: table list, tap → DataBrowserView pushes correctly with toolbar
• Query tab: SQL editor with play button and overflow menu in toolbar
• "< Connections" button → returns to connection list
• Database switcher dropdown works on both tabs
• Schema switcher appears only on Tables tab
• Safe mode indicator visible
• Query text preserved when switching tabs
• Pull-to-refresh on table list works
• Deep-link opens correct connection
• Handoff works bidirectionally
• Background → disconnect, foreground → reconnect