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/features/settings/general/General.tsx

@@ -1202,6 +1202,7 @@ function Messages() {
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile
           title="Url Preview in Encrypted Room"
+          description="URL previews in encrypted rooms are fetched by your homeserver, which sees the URL but not the message content."
           after={<Switch variant="Primary" value={encUrlPreview} onChange={setEncUrlPreview} />}
         />
       </SequenceCard>