Add the complete browser UI:
- BrowserWindow: AdwHeaderBar with navigation, tab management via
AdwTabView/AdwTabBar, find-in-page, fullscreen, zoom, D-Bus
single-instance with open/activate handlers
- Tab: WebContentView lifecycle, ViewImplementation callbacks for
title, URL, favicon, cursor, tooltips, dialogs, window management
- LadybirdBrowserWindow: GtkBuilder template widget with toolbar,
tab bar, find bar, devtools banner, and hamburger menu
- LadybirdLocationEntry: URL entry with autocomplete, domain
highlighting, and security icon
- Menu: GAction-based context menus and application menu with
keyboard accelerators
- Dialogs: JS alert/confirm/prompt (AdwAlertDialog), color picker,
file picker, select dropdown, download save dialog, toast
- GtkBuilder .ui resources for browser window, location entry
completions, and list popovers
Updates Application and main.cpp to create browser windows and
handle D-Bus activation from remote instances.
