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/components/ui/sidebar.tsx

@@ -20,7 +20,6 @@ const SIDEBAR_COOKIE_MAX_AGE = 60 * 60 * 24 * 7
 const SIDEBAR_WIDTH = '16rem'
 const SIDEBAR_WIDTH_MOBILE = '18rem'
 const SIDEBAR_WIDTH_ICON = '3rem'
-const SIDEBAR_KEYBOARD_SHORTCUT = 'b'
 
 type SidebarContextProps = {
   state: 'expanded' | 'collapsed'
@@ -87,19 +86,8 @@ function SidebarProvider({
     return isMobile ? setOpenMobile(open => !open) : setOpen(open => !open)
   }, [isMobile, setOpen, setOpenMobile])
 
-  // Adds a keyboard shortcut to toggle the sidebar.
-  React.useEffect(() => {
-    const handleKeyDown = (event: KeyboardEvent) => {
-      if (event.key === SIDEBAR_KEYBOARD_SHORTCUT && (event.metaKey || event.ctrlKey)) {
-        event.preventDefault()
-        toggleSidebar()
-      }
-    }
-
-    window.addEventListener('keydown', handleKeyDown)
-
-    return () => window.removeEventListener('keydown', handleKeyDown)
-  }, [toggleSidebar])
+  // The sidebar toggle (Cmd/Ctrl+B by default) is owned by the keybind runtime
+  // (`view.toggleSidebar`) so it appears in the hotkey map and is rebindable.
 
   // We add a state so that we can do data-state="expanded" or "collapsed".
   // This makes it easier to style the sidebar with Tailwind classes.