Add Tauri v2 + Svelte 5 frontend and headless Python backend

Scaffold the cross-platform rewrite from PySide6/Qt to Tauri + Svelte,
following the same architecture as voice-to-notes. The Python backend
runs headless as a sidecar, with a FastAPI control API that the Svelte
frontend connects to via REST and WebSocket.

New files:
- backend/app_controller.py: Headless orchestration (extracted from MainWindow)
- backend/api_server.py: FastAPI control endpoints + /ws/control WebSocket
- backend/main_headless.py: Headless entry point for sidecar mode
- src-tauri/: Tauri v2 Rust shell with sidecar and dialog plugins
- src/: Svelte 5 frontend (App, Settings, Controls, TranscriptionDisplay)
- src/lib/stores/: Reactive stores for backend connection, config, transcriptions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/App.svelte

@@ -0,0 +1,99 @@
+<script lang="ts">
+  import { onMount } from "svelte";
+  import Header from "$lib/components/Header.svelte";
+  import StatusBar from "$lib/components/StatusBar.svelte";
+  import Controls from "$lib/components/Controls.svelte";
+  import TranscriptionDisplay from "$lib/components/TranscriptionDisplay.svelte";
+  import Settings from "$lib/components/Settings.svelte";
+  import { backendStore } from "$lib/stores/backend";
+  import { configStore } from "$lib/stores/config";
+
+  let showSettings = $state(false);
+
+  let obsDisplayUrl = $derived(backendStore.obsUrl);
+  let syncDisplayUrl = $derived(backendStore.syncUrl);
+
+  function openSettings() {
+    showSettings = true;
+  }
+
+  function closeSettings() {
+    showSettings = false;
+  }
+
+  onMount(() => {
+    backendStore.connect();
+    configStore.loadConfig();
+
+    return () => {
+      backendStore.disconnect();
+    };
+  });
+</script>
+
+<div class="app-shell">
+  <Header onSettingsClick={openSettings} />
+  <StatusBar />
+
+  <div class="display-links">
+    <span class="link-label">OBS:</span>
+    <a href={obsDisplayUrl} target="_blank" rel="noopener">{obsDisplayUrl}</a>
+    {#if syncDisplayUrl}
+      <span class="link-separator">|</span>
+      <span class="link-label">Sync:</span>
+      <a href={syncDisplayUrl} target="_blank" rel="noopener"
+        >{syncDisplayUrl}</a
+      >
+    {/if}
+  </div>
+
+  <TranscriptionDisplay />
+  <Controls />
+
+  <div class="version-label">v{backendStore.version}</div>
+</div>
+
+{#if showSettings}
+  <Settings onClose={closeSettings} />
+{/if}
+
+<style>
+  .app-shell {
+    display: flex;
+    flex-direction: column;
+    height: 100%;
+    width: 100%;
+    background-color: var(--bg-primary);
+  }
+
+  .display-links {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    padding: 6px 20px;
+    font-size: 12px;
+    background-color: var(--bg-primary);
+    border-bottom: 1px solid var(--border-color);
+    flex-shrink: 0;
+  }
+
+  .link-label {
+    color: var(--text-secondary);
+    font-weight: 500;
+  }
+
+  .link-separator {
+    color: var(--text-muted);
+    margin: 0 4px;
+  }
+
+  .version-label {
+    position: fixed;
+    bottom: 6px;
+    right: 12px;
+    font-size: 11px;
+    color: var(--text-muted);
+    pointer-events: none;
+    z-index: 10;
+  }
+</style>