Download sidecar on first launch instead of bundling

Major refactor: sidecar is no longer bundled in the installer. Instead,
it's downloaded on first launch with a setup screen offering CPU vs CUDA
choice. This solves the 2GB+ installer size limit and decouples app/sidecar.

Backend:
- New commands: check_sidecar, download_sidecar, check_sidecar_update
- Streaming download with progress events via reqwest
- Added reqwest + futures-util dependencies
- Removed sidecar.zip from bundle resources
- Restored NSIS target (no longer size-constrained)

CI:
- Each platform builds both CPU and CUDA sidecar variants (except macOS: CPU only)
- Sidecar zips uploaded as separate release assets
- Asset naming: sidecar-{os}-{arch}-{variant}.zip

Frontend:
- SidecarSetup.svelte: first-launch setup with CPU/CUDA radio choice,
  progress bar, error/retry handling
- Update banner on launch if newer sidecar version available
- Conditional rendering: setup screen → main app flow

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/sidecar/mod.rs

@@ -14,7 +14,7 @@ use crate::sidecar::messages::IPCMessage;
 /// Resource directory set by the Tauri app during setup.
 static RESOURCE_DIR: OnceLock<PathBuf> = OnceLock::new();
 /// App data directory for extracting the sidecar archive.
-static DATA_DIR: OnceLock<PathBuf> = OnceLock::new();
+pub(crate) static DATA_DIR: OnceLock<PathBuf> = OnceLock::new();
 
 /// Initialize directories for sidecar resolution.
 /// Must be called from the Tauri setup before any sidecar operations.
@@ -136,7 +136,7 @@ impl SidecarManager {
     }
 
     /// Extract a zip archive to the given directory.
-    fn extract_zip(zip_path: &Path, dest: &Path) -> Result<(), String> {
+    pub(crate) fn extract_zip(zip_path: &Path, dest: &Path) -> Result<(), String> {
         eprintln!(
             "[sidecar-rs] Extracting sidecar from {} to {}",
             zip_path.display(),
@@ -185,7 +185,7 @@ impl SidecarManager {
 
     /// Remove old sidecar-* directories that don't match the current version.
     /// Called after the current version's sidecar is confirmed ready.
-    fn cleanup_old_sidecars(data_dir: &Path, current_version: &str) {
+    pub(crate) fn cleanup_old_sidecars(data_dir: &Path, current_version: &str) {
         let current_dir_name = format!("sidecar-{}", current_version);
 
         let entries = match std::fs::read_dir(data_dir) {