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/Cargo.toml

@@ -24,3 +24,5 @@ zip = { version = "2", default-features = false, features = ["deflate"] }
 thiserror = "1"
 chrono = { version = "0.4", features = ["serde"] }
 tauri-plugin-dialog = "2.6.0"
+reqwest = { version = "0.12", features = ["stream"] }
+futures-util = "0.3"