fix(composer): stop three E2EE plaintext leaks (compress, schedule, GIF)

- Image compression in an encrypted room re-encoded the *plaintext* original,
  uploaded it unencrypted, and reused the original's encInfo, so the media sat
  on the server in the clear AND the attachment was undecryptable. The
  compressed bytes are now run through encryptFile and the synthetic upload
  item carries the new encInfo (buildCompressedUploadItem, unit-tested; it can
  never inherit the stale encInfo).
- Scheduled messages (MSC4140) are PUT as raw m.room.message, bypassing the
  SDK encryption pipeline. The Schedule button is now hidden in encrypted
  rooms, handleScheduleClick no-ops there, and scheduleMessage() itself
  refuses with a clear error so no caller can regress this. README notes the
  limitation.
- The GIF picker uploaded the Giphy blob unencrypted into E2EE rooms; it now
  mirrors the voice/attachment path (encryptFile -> upload ciphertext ->
  content.file).

Fixes #6
Fixes #7
Fixes #11

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/utils/scheduledMessages.ts

@@ -12,6 +12,13 @@ export async function scheduleMessage(
   content: IContent,
   sendAtMs: number,
 ): Promise<string> {
+  // MSC4140 delayed events are PUT straight to /send/m.room.message, bypassing
+  // the SDK's encryptEventIfNeeded pipeline — the body would land on the server
+  // (and later in the timeline) in the clear. Refuse rather than leak; the
+  // composer also hides the Schedule button in encrypted rooms.
+  if (mx.getRoom?.(roomId)?.hasEncryptionStateEvent()) {
+    throw new Error('Scheduled messages are not supported in encrypted rooms.');
+  }
   // A past/near target floors at 1000ms (send ~immediately) — an intentional,
   // tested contract; the ScheduleMessageModal already guards ≥60s in the future.
   const delayMs = Math.max(1000, Math.round(sendAtMs - Date.now()));