feat(settings): storage usage tile with persistence status (#120)

One tile under Settings → General → Storage: total local usage vs the
browser's quota from navigator.storage.estimate(), the Chromium
usageDetails breakdown when available (IndexedDB = sync cache + encryption
keys + search index; Cache Storage = offline app files), and whether the
browser granted persistent storage — green "Protected" or amber "May be
evicted" with a "Keep my data" button that calls storage.persist(). That
last bit is the useful diagnostic for the KE-1 storage-eviction cluster.

No clear button: media lives in the browser's own HTTP cache (not in the
estimate, and not clearable from a page), the crypto store must never be
casually cleared, and the search-index clear already lives in Message
Search. The About page's "Clear Cache & Reload" now says it deletes this
device's encryption keys too. Hidden entirely when estimate() is missing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/settings/about/About.tsx

@@ -159,7 +159,7 @@ export function About({ requestClose }: AboutProps) {
                 >
                   <SettingTile
                     title="Clear Cache & Reload"
-                    description="Clear all your locally stored data and reload from server."
+                    description="Clear all your locally stored data and reload from server. This also deletes this device's encryption keys — older encrypted messages stay unreadable here unless key backup is set up."
                     after={
                       <Button
                         onClick={() => clearCacheAndReload(mx)}