feat(updater): self-update via GitHub Releases with launch check and settings UI

Phase 3 of the 0.1.0 release prep:
- tauri-plugin-updater + tauri-plugin-process wired into the builder, with
  updater:default / process:allow-restart capabilities
- bundle.createUpdaterArtifacts: true; endpoint points at the GitHub
  releases latest.json, pubkey baked into tauri.conf.json
- store: update status state machine (check / download progress / install)
  with a quiet launch check (prod only) that stays silent on network errors
- UI: dismissible update toast with download progress, plus an Updates group
  in Settings > General showing current version and manual check/install

src-tauri/Cargo.toml

@@ -55,6 +55,8 @@ kamadak-exif = "0.5"
 notify = "6"
 tauri-plugin-notification = "2"
 mozjpeg = "0.10.13"
+tauri-plugin-updater = "2"
+tauri-plugin-process = "2"
 
 # ── Dev-mode performance ────────────────────────────────────────────────────
 # opt-level=1 on the main crate keeps incremental compile short.