feat: in-app updates via tauri-plugin-updater

[dave6892]

Summary

• Adds tauri-plugin-updater (Rust + JS) to check for and install updates from within the app
• Settings view shows current version and an install button when a newer version is available
• CI publish-update job generates and pushes latest.json to GitHub Pages after each release build
• Signing secrets (TAURI_SIGNING_PRIVATE_KEY) passed to tauri-action so artifacts are signed

How it works

1. User opens Settings → app calls check_for_update → fetches https://fetchflow.github.io/nichinichi/latest.json
2. If a newer version exists, an "Update to vX.Y.Z — Install & Restart" button appears
3. Clicking it downloads the signed package, verifies the Ed25519 signature, applies the update, and restarts

Before merging

• Add TAURI_SIGNING_PRIVATE_KEY and TAURI_SIGNING_PRIVATE_KEY_PASSWORD as GitHub Actions secrets
• Enable GitHub Pages on the gh-pages branch (Settings → Pages)
• Confirm https://fetchflow.github.io/nichinichi/latest.json is accessible

See docs/releasing.md for the full setup and release workflow.

🤖 Generated with Claude Code