refactor: replace native window tabs with in-app tab bar

[datlechin]

Summary

• Replace native macOS window tabs (addTabbedWindow, 600-900ms per call) with a custom SwiftUI EditorTabBar for instant tab switching
• Tab restoration now loads all tabs into a single window instead of opening N separate native windows (eliminates N² view lifecycle cascades)
• All editor tab navigation (openTableTab, newTab, FK navigation, favorites, etc.) now adds in-app tabs instead of creating new NSWindow instances

Key changes

New components:

• EditorTabBar / EditorTabBarItem — custom tab bar with drag-to-reorder, close buttons, context menus, preview tab styling, dirty indicators
• MainContentCoordinator+TabOperations — tab lifecycle operations (close, reorder, rename, duplicate)

Redirected navigation (13 call sites):

• MainContentCoordinator+Navigation — 5 sites (active work guard, default open, preview paths)
• MainContentCoordinator+SidebarActions — 3 sites (createNewTable, createView, editView)
• MainContentCoordinator+FKNavigation — 1 site
• MainContentCoordinator+Favorites — 1 site
• MainContentCoordinator — 2 sites (loadQueryIntoEditor, insertQueryFromAI)
• MainContentCommandActions — 1 site (openFile)

Simplified infrastructure:

• Removed onDisappear 200ms grace period (no more tab group merge cascades)
• Simplified quit-time persistence (single coordinator saves directly, no multi-window aggregation)
• Cmd+T adds in-app tab via coordinator instead of creating native window
• Cmd+W closes in-app tab when >1 tab, closes window on last tab
• Cmd+1-9, Cmd+Shift+[/] switch in-app tabs
• Set window.tabbingMode = .disallowed (native window tabs only for connection grouping)

Test plan

• Cmd+T adds in-app tab instantly (no 600ms delay)
• Click table in sidebar opens in-app tab
• Click table with active work (unsaved changes/filters/sort) opens new in-app tab
• Cmd+W closes tab when >1 tab exists
• Cmd+W closes window on last tab
• Close tab with unsaved changes shows confirmation dialog
• Cmd+1-9 switches to Nth in-app tab
• Cmd+Shift+[ and Cmd+Shift+] cycle through tabs
• Quit with 5 tabs open → relaunch → all 5 restored in one window
• Preview tabs: single-click sidebar → italic preview tab, double-click → promoted
• Drag tabs to reorder in tab bar
• Context menu: Close, Close Others, Close to Right, Close All, Duplicate
• FK navigation opens in-app tab
• Database switch clears tabs correctly
• ER diagram, server dashboard, create table all add in-app tabs
• Multiple connections open as separate windows (each with own tab bar)