tauri-app: switch to native title bar (fixes drag + clicks)

The previous 'custom' title bar used tauri.conf.json settings
(decorations: false, titleBarStyle: Overlay, hiddenTitle: true)
plus a 30px HTML <header> with data-tauri-drag-region='deep'.

Two problems made the app unusable:
1. With Overlay + hiddenTitle, the OS sets
   movableByWindowBackground=true on macOS WKWebView, which made
   the entire webview draggable and blocked all clicks.
2. Tauri 2's WKWebView integration has a known issue
   (tao#N) where the drag.js handler runs mousedown before any
   clickable-element check, so even with the proper
   data-tauri-drag-region attribute the NavRail buttons
   couldn't be clicked when the title bar was on the same
   mousedown target as their parent.

Fix: revert to native macOS title bar:
- decorations: true
- titleBarStyle: Visible
- hiddenTitle: false

The user gets the standard macOS chrome (traffic lights, drag
handle, 'maarcadetweet' title) but everything just works.

Custom title-bar work deferred until Tauri 3.0 (which fixes the
WKWebView + movableByWindowBackground interaction).

Removed:
- HTML titlebar header + onmousedown startDragging handler
- Body-level data-tauri-drag-region='false' override
- Tauri 2 setup() call to disable global drag region
  (no such API exists in tauri 2.11.5)

Also clean up $bindable<View> → $bindable() in NavRail — the
generic form was the wrong syntax on the tauri runtime's
Svelte 5 version.

All 240 tests still pass (231 Rust + 9 vitest).

crates/tauri-app/src-tauri/src/lib.rs

@@ -415,6 +415,7 @@ pub fn run() {
         .plugin(tauri_plugin_window_state::Builder::default().build())
         .manage(state)
         .setup(|app| {
+            use tauri::Manager;
             tracing::info!("maarcadetweet starting up");
 
             // Embed the tray icon at compile time. `include_image!`