Page-requested fullscreen

[mxmgorin]

requestFullscreen() is handled by the engine but ignored by us, so a video site's fullscreen button currently does nothing on screen. On these devices, that is likely the first button a user reaches for when a <video> starts playing.

Design

• Implement WebViewDelegate::notify_fullscreen_state_changed(webview, is_fullscreen) in browser/delegate.rs, alongside the existing notify_* hooks, and keep the state per tab. A background tab entering fullscreen must not hide the chrome of the tab currently on screen.
• Fullscreen here means hiding the chrome and giving the page the whole window, not calling SDL_SetWindowFullscreen. On a handheld, the window already fills the panel; the toolbar is what costs pixels. The webview rect is the window minus the toolbar strip (ui::toolbar_height), so entering fullscreen is the same resize path already used when the toolbar auto-hides. Reuse that path rather than adding another one.
• Exiting: B calls WebView::exit_fullscreen(), while the page's own exitFullscreen() arrives through the same notification, so both paths converge in one place. While fullscreen, B must not also navigate back in history.
• Overlays such as the menu, settings, and OSK should still draw over the page. Opening one should cover fullscreen, not exit it.
• On desktop, decide whether fullscreen should also call SDL_SetWindowFullscreen. The simplest first pass is no: keep the same behaviour everywhere.

Notes

• Engine hooks: WebViewDelegate::notify_fullscreen_state_changed — webview_delegate.rs:984; WebView::exit_fullscreen — webview.rs:706.
• Add a page under tests/pages/ that requests fullscreen on click, so the path can be tested without relying on a video site.