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/tauri.conf.json

@@ -20,10 +20,10 @@
         "minWidth": 880,
         "minHeight": 540,
         "resizable": true,
-        "decorations": false,
+        "decorations": true,
         "transparent": false,
-        "titleBarStyle": "Overlay",
-        "hiddenTitle": true,
+        "titleBarStyle": "Visible",
+        "hiddenTitle": false,
         "backgroundColor": "#0D0D0D"
       }
     ],