feat(desktop): rebindable keyboard shortcuts panel

Add a central keybind registry + nanostore so desktop hotkeys are
discoverable and user-rebindable. A titlebar ⌨ button (and ⌘/) opens a
collapsible map grouped by Composer (read-only) / Profiles / Session /
Navigation / View; click any chip to capture a new combo. Overrides
persist to localStorage as a delta against shipped defaults, so future
default changes aren't shadowed by a stored snapshot.

Migrates the previously scattered inline listeners (palette, command
center, new session, sidebar, theme) into the registry, and adds profile
switch/cycle/create + default-profile hotkeys.

apps/desktop/src/app/shell/app-shell.tsx

@@ -16,6 +16,7 @@ import {
 import { $paneWidthOverride } from '@/store/panes'
 import { $connection } from '@/store/session'
 
+import { KeybindPanel } from './keybind-panel'
 import { StatusbarControls, type StatusbarItem } from './statusbar-controls'
 import { TITLEBAR_HEIGHT, titlebarControlsPosition } from './titlebar'
 import { TitlebarControls, type TitlebarTool } from './titlebar-controls'
@@ -155,6 +156,9 @@ export function AppShell({
 
       {overlays}
 
+      {/* Keybind map dialog (titlebar ⌨ button / ⌘/). */}
+      <KeybindPanel />
+
       {/* Mounted at the shell root (after overlays) so success/error toasts
           surface above every route and overlay — not just the chat view. */}
       <NotificationStack />