Purge frontend cache before first load on version change + reload-and-clear command

[bramkragten]

Part of the epic: home-assistant/epics#113 (home-assistant/frontend)

Problem

The companion app is a WKWebView with no service worker (App-Bound Domains are not configured), so it relies entirely on WKWebView's HTTP cache. After an HA update it can load a stale index.html that imports the previous build's hashed JS, which is now deleted → 404 → broken / white screen. Because there's no SW and the frontend bundle failed to load, the frontend can't self-heal on its own here.

Today's native mitigations are reactive (serverVersionDidChange) and time-based (the ~3-day cleanFrontendAssetCacheIfNeeded purge). They can lose the cold-start race — e.g. when the app and core update together and the new version was already recorded, no version-delta fires and the stale HTML loads first. (Matches #3738.)

Change

• Proactively purge the frontend asset cache before the first WKWebView load whenever the stored server version differs from the last-seen version, closing the cold-start race (rather than reacting after the stale HTML has already loaded).
• Add an external-bus command (e.g. frontend/reload_and_clear_cache) that clears the WKWebView caches and reloads, so the frontend's boot-recovery guard can delegate cache clearing to the native app. (Frontend side tracked under Frontend can fail to load after an update (stale cached index.html → deleted hashed JS 404) epics#113.)

Acceptance criteria

• Cold-launching the companion app after an HA update loads the new frontend without a manual Settings → Companion App → Debug → Reset frontend cache.

[bgoncal]

Done in #5021

[bgoncal]

Pending add frontend/reload_and_clear_cache

[bgoncal]

#5197

[bramkragten]

Done in #5197