#256 Add native fullscreen and resilient window recovery controls

[Anonymous]

Originally created by: kristofferR

Problem

Chatpane has maximize/zoom and an element-level Fullscreen API fallback, but no first-class native fullscreen action. Where a WebView's Fullscreen API is absent or incomplete, video fullscreen is simulated inside the existing decorated window.

Chatpane's Reload menu action is also page eval based, so it cannot recover a blank/error document with no working JavaScript context.

Goal

Provide native fullscreen and native recovery actions while keeping Chatpane a Messenger client rather than reintroducing browser-history shortcuts.

Scope

• Add F11 native fullscreen on Windows and Linux.
• Add the standard Full Screen item on macOS.
• Synchronize native fullscreen state with document.fullscreenElement and the standard/WebKit fullscreenchange events only where the platform WebView needs a bridge.
• Do not move or reparent React-owned media elements. Preserve the original DOM and playback state.
• Escape and external native exit must update both window and page state.
• Change Reload to a native WebView reload operation.
• Add a Messenger Home recovery action that natively navigates to the configured Messenger URL and works from an error page.
• Keep Cmd/Ctrl+[ and ] assigned to conversation navigation as decided in [#18]. Browser Back/Forward shortcuts remain out of scope.

Verification

• F11/menu fullscreen enter and exit on all platforms.
• Messenger video fullscreen, Escape, native title-bar exit, window recreation, and multiple windows.
• Reload and Home from a normal Messenger page, offline/error page, and auth/checkpoint page.
• No fullscreen-state leak after a video or window is destroyed.

Ref [#18].