feat: add Report Room (MSC4151) and fix URL preview default for encrypted rooms

- Report Room: new ReportRoomModal with reason + category, POST /rooms/{id}/report
- URL preview: encUrlPreview default changed to true; security note added to settings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/state/settings.ts

@@ -101,7 +101,7 @@ const defaultSettings: Settings = {
   hideNickAvatarEvents: true,
   mediaAutoLoad: true,
   urlPreview: true,
-  encUrlPreview: false,
+  encUrlPreview: true,
   showHiddenEvents: false,
   legacyUsernameColor: false,