feat: add in-app auto-update via Tauri updater plugin

Integrates tauri-plugin-updater and tauri-plugin-process to enable
ClawPal self-updates. The Home page now checks for app updates and
shows a download progress bar with an "Update & Restart" button.
CI workflow wired to sign release artifacts with TAURI_SIGNING_PRIVATE_KEY.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

src-tauri/capabilities/default.json

@@ -0,0 +1,5 @@
+{
+  "identifier": "default",
+  "windows": ["main"],
+  "permissions": ["core:default", "updater:default", "process:default"]
+}