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/lib.rs

@@ -14,6 +14,7 @@ use commands::project::{
     load_project_transcript, save_project_file, save_project_transcript, update_segment,
 };
 use commands::settings::{load_settings, save_settings};
+use commands::sidecar::{check_sidecar, check_sidecar_update, download_sidecar};
 use commands::system::{get_data_dir, llama_list_models, llama_start, llama_status, llama_stop};
 use commands::transcribe::{download_diarize_model, run_pipeline, transcribe_file};
 use state::AppState;
@@ -65,6 +66,9 @@ pub fn run() {
             get_data_dir,
             load_settings,
             save_settings,
+            check_sidecar,
+            download_sidecar,
+            check_sidecar_update,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");