refactor: unify new tab creation — TabIntent, pendingPayloads, native tab bar fix

[datlechin]

Summary

• Replace race-prone pendingConnectionId singleton with pendingPayloads dictionary (payload ID → connection ID) — supports parallel window opens without races
• Add TabIntent enum (.openContent, .newEmptyTab, .restoreOrDefault) to EditorTabPayload, replacing overlapping isNewTab / isConnectionOnly semantics
• Fix native macOS tab bar "+" button via newWindowForTab: override on AppDelegate
• Unify all 6 direct openWindow(id: "main", ...) call sites through WindowOpener.openNativeTab()
• Intent-based routing in SessionStateFactory and initializeAndRestoreTabs — clean switch instead of fragile fallthrough
• Remove 100ms + 50ms timing hacks in tab restore (no longer needed with set-based tracking)
• Fix ghost toolbar pill: set isTableTab = true in factory for .table payloads
• Fix sidebar empty flash: show loading spinner when connection is active but tables haven't loaded yet
• Resolve session synchronously for nil-payload windows (native tab bar creates ContentView(payload: nil))

Test plan

• Cmd+T opens new empty query tab in correct tab group
• Toolbar "+" button: same behavior as Cmd+T
• Native tab bar "+" button: creates proper new tab (was broken before — created invisible orphan)
• Open table (double-click sidebar): opens in new tab with correct toolbar state
• Connect from Welcome window: window opens with correct tab group
• Connect from Dock menu: same
• Connection Switcher: switching connections opens in correct window/tab group
• Tab restore on app launch: all tabs restore without timing-dependent flicker
• Rapid Cmd+T: multiple tabs opened quickly don't race on connection assignment
• Build succeeds: xcodebuild -scheme TablePro build -skipPackagePluginValidation