feat: add collaboration settings and host policy sync

[simonwinther]

What changed

• adds a Collaboration settings tab for hosting, copying invites, joining, leaving, and live room status over direct local IPC
• makes the host authoritative for manual resume, notification timing, and strict coordination policy while keeping each participant's visual settings local
• forwards manual-resume input from any guest reliably at the shared deadline without weakening normal-mode validation
• sizes and wraps overlay content to each local monitor, including compact layouts and vertical scrolling
• documents the host/local policy boundary and Tailscale MagicDNS workflow

Why

Co-op setup previously required CLI commands, guest notification and strict behavior could conflict with local configuration, and a guest's zero-second resume input could race the host deadline. Fixed-width overlay content could also overflow smaller monitors.

Impact

Users can manage rooms entirely from the GTK settings window. Co-op participants follow one coordination policy while retaining local opacity, monitor, messages, sound, and other presentation choices. The settings poll uses IPC rather than spawning recurring processes, and co-op remains inactive-cost-free when disabled.

Validation

• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo build --locked --release --workspace
• GTK development-instance smoke test of Schedule, Host, and Join views