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/App.svelte

@@ -465,19 +465,26 @@
 {/if}
 
 <style>
+  /* The whole page is a flex column: 30px title bar on top, then the
+     rest of the content. The title bar is the only drag region; the
+     rest is clickable thanks to Tauri's drag.js clickable-element
+     check. */
   .login-wrap {
-    height: 100%;
+    flex: 1;
     display: grid;
     place-items: center;
+    min-height: 0;
   }
+
   .shell {
+    flex: 1;
     display: grid;
     grid-template-rows: 1fr 24px;
     grid-template-columns: 88px 1fr;
     grid-template-areas:
       "rail main"
       "rail status";
-    height: 100%;
+    min-height: 0;
   }
   .shell > :global(nav.rail) { grid-area: rail; }
   .shell > :global(.statusbar) { grid-area: status; }