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/tauri.conf.json

@@ -31,7 +31,7 @@
   },
   "bundle": {
     "active": true,
-    "targets": ["deb", "rpm", "msi", "dmg"],
+    "targets": ["deb", "rpm", "nsis", "dmg"],
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",
@@ -42,7 +42,7 @@
     "category": "Utility",
     "shortDescription": "Transcribe audio/video with speaker identification",
     "longDescription": "Voice to Notes is a desktop application that transcribes audio and video recordings with speaker identification, synchronized playback, and AI-powered analysis. Export to SRT, WebVTT, ASS captions, or plain text.",
-    "resources": ["sidecar.zip"],
+    "resources": [],
     "copyright": "Voice to Notes Contributors",
     "license": "MIT",
     "linux": {