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/Cargo.toml

@@ -15,5 +15,7 @@ tauri-build = { version = "2", features = [] }
 [dependencies]
 tauri = { version = "2", features = ["tray-icon", "image-png"] }
 tauri-plugin-single-instance = "2"
+tauri-plugin-updater = "2"
+tauri-plugin-process = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"