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-tauri/src/lib.rs

@@ -1,9 +1,41 @@
+mod sidecar;
+
+use std::sync::Mutex;
+use tauri::Manager;
+
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
     tauri::Builder::default()
         .plugin(tauri_plugin_shell::init())
         .plugin(tauri_plugin_dialog::init())
         .plugin(tauri_plugin_process::init())
+        .manage(sidecar::ManagedSidecar(Mutex::new(
+            sidecar::SidecarManager::new(),
+        )))
+        .setup(|app| {
+            let resource_dir = app
+                .path()
+                .resource_dir()
+                .expect("failed to resolve resource dir");
+            let data_dir = app
+                .path()
+                .app_data_dir()
+                .expect("failed to resolve app data dir");
+
+            // Ensure the data directory exists
+            std::fs::create_dir_all(&data_dir).expect("failed to create app data dir");
+
+            sidecar::init_dirs(resource_dir, data_dir);
+            Ok(())
+        })
+        .invoke_handler(tauri::generate_handler![
+            sidecar::check_sidecar,
+            sidecar::download_sidecar,
+            sidecar::check_sidecar_update,
+            sidecar::get_sidecar_port,
+            sidecar::start_sidecar,
+            sidecar::stop_sidecar,
+        ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");
 }