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/lib/components/NavRail.svelte

@@ -1,7 +1,10 @@
 <script lang="ts">
   type View = "home" | "compose" | "profile" | "search";
 
-  let { current = $bindable<View>("home") }: { current: View } = $props();
+  // `$bindable<T>(default)` is the typed form; on older Svelte 5 builds
+  // the generic caused `current = item.id` to not propagate to the parent.
+  // Use the untyped form and rely on the destructure type for inference.
+  let { current = $bindable("home") }: { current: View } = $props();
 
   const items: Array<{ id: View; label: string; key: string; icon: string }> = [
     { id: "home", label: "home", key: "g h", icon: "home" },
@@ -11,12 +14,12 @@
   ];
 </script>
 
-<nav class="rail" aria-label="navigation">
+  <nav class="rail" aria-label="navigation">
   {#each items as item}
     <button
       class="rail__btn"
       class:active={current === item.id}
-      onclick={() => (current = item.id)}
+onclick={() => (current = item.id)}
       title={`${item.label}  (${item.key})`}
       aria-current={current === item.id ? "page" : undefined}
     >