cleanup: remove all dead AppState.shared writes and delete AppState class

[datlechin]

Summary

Removes all 51 dead AppState.shared write statements and deletes the AppState class definition. After PR #610 migrated all reads to @FocusedValue, these writes had zero readers.

11 files, -94 lines:

• TableProApp.swift — deleted AppState class (15 lines)
• ContentView.swift — removed 22 writes (connection state push on session/key window changes)
• MainContentCoordinator+Navigation.swift — removed 7 writes
• MainContentView+EventHandlers.swift — removed 4 writes
• MainContentCoordinator+TabSwitch.swift — removed 4 writes
• TableStructureView.swift — removed 4 writes
• MainContentCoordinator+FKNavigation.swift — removed 2 writes
• MainContentCoordinator+QueryHelpers.swift — removed 2 writes
• MainContentView.swift — removed 1 write
• MainEditorContentView.swift — removed 1 write
• TableStructureView+DataLoading.swift — removed 1 write

grep -rn "AppState" TablePro/ --include='*.swift' returns zero results.

Test plan

• Build succeeds
• All menu items still enable/disable correctly
• Multi-window: menus reflect active window