perf: cache selectedTabIndex + @ObservationIgnored on internal caches

[datlechin]

Summary

• selectedTabIndex and selectedTab were O(n) computed properties scanning the tabs array via first { }. Called 160+ times per body evaluation cycle across 24 files (binding setters, coordinator extensions, view bodies). With 20+ tabs, this accumulated into measurable lag during column resize and sort.
• Replaced with a lazy [UUID: Int] dictionary lookup, rebuilt only when the tabs array mutates via didSet. Amortizes the scan to once per mutation instead of once per access.
• Marked querySortCache and isSwitchingDatabase as @ObservationIgnored on MainContentCoordinator — both are internal state never read from views, but were broadcasting change notifications on every write.

Test plan

• Open 10+ tabs, switch between them — no regression
• Open a table with 20+ columns, rapidly resize columns — should feel snappier
• Sort a large query result (1000+ rows) — verify sort still works
• Switch databases — verify no regression