feat: optimize profile image sizes, silent PWA updates, storage cleanup fixes

- Resize avatars/icons to 256px and banners to 1280px (client crop + server safety net)
- Add server-side resizeProfileImage() for federation/API uploads without crop modal
- Fix unconstrained crop on RegisterPage and CreateSpace (was missing maxOutputDimension)
- PWA: switch to autoUpdate with skipWaiting/clientsClaim for seamless deploys
- Storage janitor: exclude profile images from unlinked cleanup, delete stale thumbnails
- Add deleteAttachmentByFilename() to clean orphaned attachment records for profile images
- Migration: one-time cleanup of stale profile image attachment records
- GeneralPanel: wrap in <form> to prevent implicit submission

packages/web/src/components/modals/instanceSettingsPanels/GeneralPanel.tsx

@@ -66,7 +66,7 @@ export function GeneralPanel() {
   };
 
   return (
-    <div className="space-y-5">
+    <form className="space-y-5" onSubmit={(e) => e.preventDefault()}>
       <div className="text-xs text-txt-tertiary">
         Configure your Backspace instance. These settings affect all users.
       </div>
@@ -178,6 +178,6 @@ export function GeneralPanel() {
           </div>
         </div>
       )}
-    </div>
+    </form>
   );
 }