Add sidecar download, setup screen, and auto-launch

On first launch, the app now prompts users to download the Python
sidecar (CPU or CUDA variant) from Gitea releases, matching the
voice-to-notes pattern. On subsequent launches, it auto-launches
the sidecar and connects.

New Rust module (src-tauri/src/sidecar/):
- download_sidecar: streams download with progress events, extracts zip
- check_sidecar: verifies installed sidecar binary exists
- check_sidecar_update: compares local vs latest release version
- SidecarManager: launches binary, waits for ready JSON, manages lifecycle
- Dev mode: runs `python -m backend.main_headless` directly
- start_sidecar/stop_sidecar/get_sidecar_port: Tauri commands

New Svelte component (SidecarSetup.svelte):
- First-time setup overlay with CPU/CUDA variant selection
- Download progress bar with byte counter
- Error state with retry, success state with auto-continue

Updated App.svelte state machine:
- checking -> needs_setup -> starting -> connected
- Falls back to direct connection in browser dev mode

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

src/App.svelte

@@ -5,10 +5,14 @@
   import Controls from "$lib/components/Controls.svelte";
   import TranscriptionDisplay from "$lib/components/TranscriptionDisplay.svelte";
   import Settings from "$lib/components/Settings.svelte";
+  import SidecarSetup from "$lib/components/SidecarSetup.svelte";
   import { backendStore } from "$lib/stores/backend";
   import { configStore } from "$lib/stores/config";
 
+  type SidecarState = "checking" | "needs_setup" | "starting" | "connected";
+
   let showSettings = $state(false);
+  let sidecarState = $state<SidecarState>("checking");
 
   let obsDisplayUrl = $derived(backendStore.obsUrl);
   let syncDisplayUrl = $derived(backendStore.syncUrl);
@@ -23,9 +27,55 @@
     showSettings = false;
   }
 
+  async function checkAndLaunchSidecar() {
+    try {
+      const { invoke } = await import("@tauri-apps/api/core");
+
+      // Check if sidecar is installed
+      sidecarState = "checking";
+      const installed = await invoke<boolean>("check_sidecar");
+
+      if (!installed) {
+        sidecarState = "needs_setup";
+        return;
+      }
+
+      await launchSidecar();
+    } catch {
+      // Not running in Tauri (browser dev mode) - skip sidecar check
+      // and connect directly to localhost:8081
+      sidecarState = "starting";
+      backendStore.setPort(8081);
+      backendStore.connect();
+      configStore.loadConfig();
+    }
+  }
+
+  async function launchSidecar() {
+    try {
+      const { invoke } = await import("@tauri-apps/api/core");
+
+      sidecarState = "starting";
+      await invoke("start_sidecar");
+
+      const port = await invoke<number>("get_sidecar_port");
+      backendStore.setPort(port);
+      backendStore.connect();
+      configStore.loadConfig();
+    } catch {
+      // If sidecar launch fails, still try connecting to default port
+      sidecarState = "starting";
+      backendStore.connect();
+      configStore.loadConfig();
+    }
+  }
+
+  async function onSidecarReady() {
+    await launchSidecar();
+  }
+
   onMount(() => {
-    backendStore.connect();
-    configStore.loadConfig();
+    checkAndLaunchSidecar();
 
     return () => {
       backendStore.disconnect();
@@ -33,7 +83,21 @@
   });
 </script>
 
-{#if !isConnected}
+{#if sidecarState === "checking"}
+  <div class="connecting-overlay">
+    <div class="connecting-content">
+      <div class="connecting-icon">
+        <div class="spinner"></div>
+      </div>
+      <h2>Local Transcription</h2>
+      <p>Checking setup...</p>
+    </div>
+  </div>
+
+{:else if sidecarState === "needs_setup"}
+  <SidecarSetup onComplete={onSidecarReady} />
+
+{:else if !isConnected}
   <div class="connecting-overlay">
     <div class="connecting-content">
       <div class="connecting-icon">
@@ -57,6 +121,7 @@
       {/if}
     </div>
   </div>
+
 {:else}
   <div class="app-shell">
     <Header onSettingsClick={openSettings} />