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/chat/composer/help-hint.tsx

@@ -5,7 +5,7 @@ import { useI18n } from '@/i18n'
 import { COMPLETION_DRAWER_CLASS } from './completion-drawer'
 
 const COMMON_COMMAND_KEYS = ['/help', '/clear', '/resume', '/details', '/copy', '/quit']
-const HOTKEY_KEYS = ['@', '/', '?', 'Enter', 'Cmd/Ctrl+K', 'Cmd/Ctrl+L', 'Esc', '↑ / ↓']
+const HOTKEY_KEYS = ['@', '/', '?', 'Enter', 'Cmd/Ctrl+Shift+K', 'Cmd/Ctrl+/', 'Esc', '↑ / ↓']
 
 export function HelpHint() {
   const { t } = useI18n()