feat(ie-shell): wire keyboard-driven UI with overlays and navigation #8

- keybindings.rs: Action enum + resolve_keybinding for all shortcuts
  (Ctrl+L/T/W/D/Q, Ctrl+Shift+T/B, Ctrl+Tab, Alt+Left/Right, Escape)
- overlay.rs: OverlayState (None/AddressBar/TabList/Bookmarks) with
  AddressBarState supporting full cursor editing and Unicode
- app.rs: Browser struct wired with TabManager, BookmarkStore,
  NavigationService, tokio runtime. Async navigation via
  EventLoopProxy<UserEvent>. Keyboard dispatch, address bar submit
  with https:// prepending, modifier tracking.
- All state changes logged via tracing::info!
- 75 unit tests + 2 smoke integration tests

Author: thomasnemer