Add explicit update checks and website links

[JHoff1]

What changed

• Add an explicit, user-triggered GitHub release check to the desktop status bar.
• Display checking, up-to-date, new-version, and unavailable states.
• Open the latest release only when a user selects the available-update state.
• Restrict network permission to GitHub's single latest-release endpoint.
• Add the live VerityPDF website to Preferences, README, package metadata, Cargo metadata, and Store documentation.
• Update privacy language to describe the opt-in request accurately.

Privacy and user impact

No update request occurs during startup or in the background. The request retrieves only public release metadata and contains no document content, filenames, paths, preferences, or diagnostics.

Validation

• Production Vite build
• 25 Vitest unit tests
• 28 Playwright regression tests
• Native Tauri cargo check --locked
• git diff --check