Split CI into app + sidecar workflows, fix reqwest compilation

CI split:
- release.yml: version bump + lightweight app builds (no Python/sidecar)
- build-sidecar.yml: builds CPU + CUDA sidecar variants per platform,
  uploads as separate release assets, runs in parallel with app builds
- Sidecar workflow uses retry loop to find release (race with version bump)

Fixes:
- Add reqwest "json" feature for .json() method
- Add explicit type annotations for reqwest Response and bytes::Bytes
- Reuse client instance for download (was using reqwest::get directly)

Bundle targets: deb, rpm, nsis, msi, dmg (all formats, app is small now)
Windows upload finds both *.msi and *-setup.exe

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

src-tauri/Cargo.toml

@@ -24,5 +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"] }
+reqwest = { version = "0.12", features = ["stream", "json"] }
 futures-util = "0.3"