Add session fullscreen shortcut and auto-toggle on connection

[zortos293]

This PR adds a configurable fullscreen shortcut that works only while in an active session and fixes auto-fullscreen to enter when connecting rather than only on pointer lock.

Settings & Persistence:

• Added shortcutToggleFullscreen to shared and main settings interfaces with default F10.
• Added to DEFAULT_SHORTCUTS in renderer and defaults in SettingsPage.

Renderer Behavior:

• Introduced setSessionFullscreen/toggleSessionFullscreen helpers that unify HTML fullscreen requests with native window fullscreen sync (via window.openNow.setFullscreen). Used by pointer-lock auto-fullscreen, main-process events, manual toggle, and the new auto-connect effect.
• Added toggleFullscreen to the global keydown handler, gated to streamStatus === "connecting" || "streaming" with prevent/stop/stopImmediatePropagation like other session shortcuts.
• Added auto-fullscreen effect that enters fullscreen once when transitioning to connecting or streaming, guarded by autoFullscreenRequestedRef to avoid repeated calls.
• Removed the late-in-stream auto-fullscreen code from the signaling offer handler; it now happens earlier via the connection effect.
• Passed toggleFullscreen into StreamView.shortcuts prop and added a "Full screen" hint to the on-screen shortcuts overlay.

Settings Page:

• Added local state (toggleFullscreenInput, toggleFullscreenError) with effect sync.
• Added validation/capture/reset logic in handleShortcutBlur, applyShortcutCapture, handleShortcutPaste, handleShortcutCaptureKeyDown, areShortcutsDefault, handleResetShortcuts.
• Added a new shortcut row labeled "Toggle Full Screen" with full binding/blur/paste/capture handling.
• Updated the error aggregation and hint text to include the fullscreen shortcut example.
• Updated the autoFullScreen hint to: "Automatically enter fullscreen when connecting to or starting a session."

Main Process:

• Removed startup auto-fullscreen initialization (the old behavior that forced fullscreen on app launch if enabled).
• Removed the autoFullScreen settings-change handler that immediately toggled native fullscreen; fullscreen is now driven by renderer session logic.