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/Cargo.toml

@@ -19,3 +19,8 @@ tauri-plugin-dialog = "2"
 tauri-plugin-process = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+reqwest = { version = "0.12", features = ["json", "stream"] }
+futures-util = "0.3"
+zip = { version = "2", default-features = false, features = ["deflate"] }
+bytes = "1"
+tokio = { version = "1", features = ["full"] }