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/capabilities/default.json

@@ -14,6 +14,8 @@
     "fs:read-files",
     "fs:read-dirs",
     "notification:default",
+    "updater:default",
+    "process:allow-restart",
     "core:window:allow-minimize",
     "core:window:allow-close",
     "core:window:allow-toggle-maximize",