refactor: remove tab bar — single note open at a time

Replace the multi-tab model with single-note navigation. One note is
open at a time; clicking any note (sidebar, wikilink, relationship)
replaces the current note in the editor. Back/Forward history still
works via Cmd+[/].

Removed: TabBar component, useClosedTabHistory, tab reorder/close/
reopen logic, Cmd+W/Cmd+Shift+T shortcuts, Close Tab and Reopen
Closed Tab menu items, tabLayout utility, and all related tests.

Simplified: useTabManagement (single-note), useAppNavigation (no tab
lookup), useKeyboardNavigation (note nav only), useNoteCreation (no
close-tab cleanup), useDeleteActions (deselect instead of close tab),
Editor (no TabBar render), Rust menu (removed 2 menu items).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/utils/tabLayout.ts

@@ -1,5 +0,0 @@
-/** Compute per-tab max-width so all tabs fit within the container. */
-export function computeTabMaxWidth(containerWidth: number, tabCount: number): number {
-  if (tabCount === 0) return 360
-  return Math.max(60, Math.min(360, Math.floor(containerWidth / tabCount)))
-}