Part of the Godot→Flutter migration (#79). Low priority / nice-to-have.

Current state

On web the updater can't offer a download; it just tells the user to refresh: platformAssetUrl() returns null for web (update_controller.dart:185-211) and the UI prompts a manual page refresh. A user on a stale tab keeps running the old build until they reload.

What's needed

• Detect when a new build is deployed (service worker updatefound / new asset hash) for the daccord-web.zip deployment.
• Surface an in-app "New version available — reload" prompt that triggers skipWaiting + reload, instead of relying on the user to refresh manually.

Acceptance

A deployed web update prompts active sessions to reload into the new build without a manual hard-refresh.

Migrated from DaccordProject/daccord-app#84