docs(desktop): document HUD mode — long-press to move, resize, snap, exit

The desktop docs had no HUD mode section at all. Add one covering the
key interaction (long-press the composer to drag the bar), plus resize,
snap-to-pointer, and exit — all sourced from the actual implementation
in composer-drag.ts, resize-handle.ts, and keybinds/actions.ts.

website/docs/user-guide/desktop.md

@@ -127,6 +127,15 @@ Quick Entry is a small always-available composer summoned by a **global hotkey f
 
 Talk to Hermes and hear it back, the same [voice mode](./features/voice-mode.md) available elsewhere. On macOS the OS will prompt once for microphone access.
 
+### HUD mode
+
+**⌘/Ctrl+Shift+H** (or the titlebar button) detaches the chat into a chrome-free, always-on-top floating bar that sits over whatever you are working in. The app window steps aside; the HUD keeps your live conversation and a composer. Where you park it is context — the bar's position tells Hermes which app and screen you're asking about, so "this", "here", and "that page" resolve to what's underneath it.
+
+- **Moving the bar** — **press and hold** anywhere on the composer for a beat, then drag. A quick press still types; a held press grabs the window. This is the only way to move the HUD — there is no titlebar to drag.
+- **Resizing** — drag the bottom-right corner of the bar.
+- **Snap to pointer** — **⌘/Ctrl+Shift+G** (a global hotkey, works from any app) jumps the HUD to wherever your cursor is.
+- **Exiting** — click the exit button on the bar, or press **⌘/Ctrl+Shift+H** again. The app window comes back with your session intact.
+
 ### Settings & onboarding
 
 Manage providers, models, tools, and credentials from a real UI instead of editing YAML. First-run onboarding gets you to your first message in seconds. The settings panes cover providers/keys, model selection, toolset configuration, MCP servers, the gateway, and session management.