BUG: Production backend/sync sidecars only spawn under the `ci` Cargo feature - misnamed flag is a silent footgun

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

In `frontend/src-tauri/src/main.rs`, the `prod()` function that spawns the bundled Python sidecars (`PictoPy_Server`, `PictoPy_Sync`) is gated behind `#[cfg(feature = "ci")]` (line 105); the `#[cfg(not(feature = "ci"))]` variant (line 186) is a **no-op**. Release builds work today because the workflows pass `--features ci` - but the name implies "continuous integration", so anyone building a release without that flag gets an app that launches and **silently never starts its backend**, with no error.

**Fix:** rename the feature to something descriptive (e.g. `bundled-sidecars`) and/or make a release build without it fail loudly. Feature defined at `frontend/src-tauri/Cargo.toml:44`.

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