feat(ux/a11y): visible, labeled command-palette trigger in the sidebar

A ⌘K palette with no visible entry point is itself a discoverability +
a11y gap (keyboard/SR users can't find it). Added a labeled 'Jump to…'
button (aria-label spells out the shortcut) that dispatches a
'commandpalette:open' event the palette now also listens for. Verified
the other top a11y items are already satisfied here (sonner Toaster
provides the aria-live region; sidebar icon buttons are aria-labeled;
skip-link + drawer focus mgmt already present).

components/command-palette.tsx

@@ -96,8 +96,13 @@ export function CommandPalette({
         setOpen(o => !o)
       }
     }
+    const onOpen = () => setOpen(true)
     document.addEventListener('keydown', onKey)
-    return () => document.removeEventListener('keydown', onKey)
+    window.addEventListener('commandpalette:open', onOpen)
+    return () => {
+      document.removeEventListener('keydown', onKey)
+      window.removeEventListener('commandpalette:open', onOpen)
+    }
   }, [])
 
   const allowed = useCallback(