feat(desktop): auto update

[elijah-potter]

Issues

N/A

Description

This PR adds Harper Desktop update support and improves a few related desktop onboarding flows.

Harper Desktop Updates

• Configures Tauri updater artifacts and the desktop updater endpoint in harper-desktop/src-tauri/tauri.conf.json.
• Adds updater permissions/dependencies for the desktop app.
• Adds DesktopUpdater, a centralized frontend update helper for:
  • Reading the current app version,
  • Reading the latest published version,
  • Running manual updates,
  • Running silent daily auto-update checks.
• Persists auto-update settings in desktop config:
  • auto_update
  • last_update_check
• Wires the General settings page to:
  • Toggle automatic updates,
  • Show current/latest versions,
  • Manually check for updates.
• Replaces the hardcoded About-page version with the packaged Tauri app version.

Desktop Release Workflow

• Updates .github/workflows/desktop.yml to build only macOS desktop artifacts for now.
• Adds Tauri signing secrets only to tagged release builds.
• Uploads macOS updater bundles and .sig files to both workflow artifacts and draft GitHub releases.

Web Updater Endpoint

• Extends the GitHub release client to fetch and cache full release metadata.
• Adds /download-harper-desktop/[target]/[arch]/[current_version] for Tauri dynamic updater metadata.
• Serves updater JSON pointing at GitHub release assets.
• Caches fetched .sig contents in memory.
• Updates /latestversion to return the release tag version and allow Desktop requests.

Demo

N/A

How Has This Been Tested?

• Added/updated Rust unit coverage for AnchorEnd, SequenceExpr, Addicting, phrase-set corrections, regionalisms, and desktop config serialization defaults.

AI Disclosure

• I am a human and didn't use any AI.
• I used LLM features of my editor, but not an agent.
• I used an AI agent interactively.
• I am an agent or I got an agent to do the work autonomously.

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have considered splitting this into smaller pull requests.