feat: Desktop auto-updater + release SBOMs

- tauri-plugin-updater + process: pubkey embedded in tauri.conf.json,
  createUpdaterArtifacts, endpoint = GitHub latest.json manifest.
- Connect screen checks before auto-connect (bounded 5s) — a saved
  URL would otherwise navigate away before the check resolves.
  "Update & restart" downloads, verifies minisign, installs, relaunches.
- Release workflow signs updater artifacts with
  TAURI_SIGNING_PRIVATE_KEY secrets, ships the .sig files, and assembles
  latest.json from them. No key → no manifest → no update, never a
  silent unsigned install.
- anchore/sbom-action emits CycloneDX SBOMs per GHCR image plus the
  source tree, attached to every release.

Note: the signing keypair was generated locally
(~/.tauri/primora-updater.key); the private half must be added to repo
secrets before the next release for updates to activate.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/desktop/src-tauri/src/lib.rs

@@ -200,6 +200,8 @@ pub fn run() {
         .plugin(tauri_plugin_single_instance::init(|app, _args, _cwd| {
             show_main(app);
         }))
+        .plugin(tauri_plugin_updater::Builder::new().build())
+        .plugin(tauri_plugin_process::init())
         .invoke_handler(tauri::generate_handler![
             get_server_url,
             set_server_url,