Proposal: Cmd/Ctrl-K command palette for the dashboard

[mvanhorn]

Linear, Notion, GitHub, Raycast all use Cmd-K as a productivity baseline. SurfSense's dashboard is action-heavy (search spaces, connectors, documents, chats, memory, LLM roles) and currently requires a lot of sidebar clicks. No open-source NotebookLM-alternative has shipped a palette (AnythingLLM, Khoj, Open-Notebook: no; Onyx: limited).

I'd add a global <CommandPalette /> at the dashboard layout, opened by Cmd/Ctrl-K, with 5 action groups: Navigation, Search Spaces, Documents (recent 10), Chats (recent 10), Quick Actions (New Chat, Upload, Settings, Toggle Theme, Log Out). Built on shadcn's CommandDialog (cmdk primitive — likely already in the tree via shadcn init).

Scope: ~250 LOC, pure frontend, no backend changes. Bundle delta ≤2KB if cmdk is already imported.

Two decisions:

1. Cmd-K is the obvious shortcut, but conflicts with some browsers' default "focus address bar" — is there a preferred secondary shortcut to standardize on? (Most apps accept this collision; users hit Cmd-K on Cmd-K apps.)
2. Should the palette be available only inside /dashboard/**, or also on landing pages? I lean dashboard-only.

Shipped #1215 / #1220 / #1233. Targeting dev, screenshot + demo GIF in PR body.