feat: add Quick Prompt global hotkey popup for one-off AI questions

Add a system-wide global hotkey (Win+Shift+A / Cmd+Shift+A) that summons
a floating, frameless popup window for quick AI interactions from any app.

Main process:
- GlobalHotkeyService for registering/unregistering system-wide shortcuts
- Frameless, transparent, always-on-top BrowserWindow factory
- IPC handlers for send, discard, close, continue-in-aryx, cancel
- Session event routing from sidecar to quick prompt window
- Settings persistence for default model, hotkey, and reasoning effort

Renderer (separate lightweight entry):
- QuickPromptApp with session state, streaming, keyboard shortcuts
- QuickPromptInput with model selector trigger and cancel support
- QuickPromptResponse with streamed markdown and thinking blocks
- QuickPromptActions (Discard / Close / Continue in Aryx)
- ModelSelector dropdown with tier badges and reasoning effort
- Glass Command Bar aesthetic with animated gradient border

Settings:
- Quick Prompt section in SettingsPanel with enable toggle, hotkey
  display, default model selector, and reasoning effort picker

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

README.md

@@ -45,6 +45,7 @@ Aryx is a desktop app that turns GitHub Copilot into a full workspace. Connect r
 | Message actions | Copy, pin, edit-and-resend, and regenerate individual messages |
 | Bookmarks | Browse all pinned messages across sessions in one panel (`Ctrl+Shift+B`) |
 | System tray | Minimize to tray, quick-launch scratchpads, and see running session count |
+| Quick Prompt | System-wide hotkey (`Win+Shift+A` / `Cmd+Shift+A`) summons a floating popup for instant AI questions from any app |
 | Desktop notifications | Native OS alerts when runs complete, fail, or need approval |
 | Onboarding | First-launch walkthrough, interactive tooltips, and a "try it" quickstart |