feat: Add opt-in Discord Rich Presence

[kylejschultz]

Summary

Add an opt-in, desktop-only Discord Rich Presence integration for local Prism playback.

Changes

• Add a small Tauri IPC adapter for Discord’s local Rich Presence protocol.
• Add a Privacy settings toggle, disabled by default.
• Update presence for track changes, pause/resume, and seeks; clear it when disabled, local playback ends, or Prism exits.
• Gracefully no-op when Discord is closed or unavailable.
• Document the privacy behavior.

Validation

• npm run test
• npm run build
• cargo check
• Preview refreshed at http://10.10.10.11:1420/ from feat/discord-rich-presence at 222742b.

Follow-up

Upload Prism’s application icon to the Discord Developer Portal with the asset key prism-player to enable the branded Rich Presence image. No client secret is needed.

Rollback

Revert commit 222742b to remove the opt-in integration.