Add a setting to hide the title bar

[calops]

Hide the title bar is now persisted with preferences. On Linux and Windows it takes effect next time Janis opens: main is constructed after preferences load, so GTK receives decorations = false before the window is realized. The app header then exposes minimize, maximize/restore and close. macOS keeps titleBarStyle: Overlay, so the toggle is not shown there.

Migration 4 adds hide_title_bar; the preference is part of get_preferences and set_title_bar_hidden. The settings UI has English and French strings, and docs/specs.md and CLAUDE.md describe the new command and behavior.

Verified with cargo fmt --check, cargo test (6 passed), debug and release cargo clippy -- -D warnings, pnpm check, pnpm test:unit (8 passed), targeted Prettier (changed code and specs), and a pnpm tauri dev smoke launch. The repository-wide Prettier check still reports the existing CLAUDE.md formatting mismatch. The commit is unsigned because 1Password's agent would not sign from this session.

[obazin]

Thanks for this — the feature itself is solid, and the persistence plus the in-app min/max/close controls are exactly right. My one substantive concern is the apply mechanism in main.rs, which I think can be much simpler.

The whole window reconstruction — create = false, rebuilding via WebviewWindowBuilder::from_config, manually re-injecting the dev URL, the visible=false → show() dance — hangs on this premise:

GTK only applies decorations before a window is realized

I don't think that holds for Tauri v2. Both getCurrentWindow().setDecorations(boolean) (JS) and WebviewWindow::set_decorations(bool) (Rust) toggle decorations on a live window, and neither documents a Linux/GTK "must set before realize" caveat (tao maps it to gtk_window_set_decorated, which takes effect at runtime).

If that's right, we can leave the window declared in tauri.conf.json as-is, delete the entire main.rs block, and apply the preference at runtime instead: in +layout.svelte's existing bootPromise (it already loads preferences), if (!isMac) getCurrentWindow().setDecorations(!prefs.hideTitleBar), and have the settings toggle call setDecorations(...) too. That removes the fragile Rust (no create=false, no from_config rebuild, no dev-URL re-injection, no read_preferences in setup) and, as a bonus, makes the toggle take effect immediately instead of on next launch.

The one thing worth confirming first: could you test setDecorations live on Linux/GTK on your side? The docs say it's unconditional, but you clearly hit something that pushed you toward construction-time. If it genuinely flickers or no-ops on some WM there, the construction-time approach is defensible and I'm happy to take the PR closer to as-is. On macOS it's moot (overlay style, the toggle is hidden) and on Windows runtime toggling is fine.

For completeness: decorations: false is also a static window option in tauri.conf.json, but since we want a per-user toggle rather than always-off, the runtime call is the right tool rather than the JSON one.