fix(media-gallery): address review — proper decrypt-download + mimetype sanitize

Review of 28cb004e (3 agents) surfaced two real issues:
- Downloads used a raw mxc→http anchor, so encrypted media (voice messages are
  almost always in E2EE DMs) downloaded ciphertext. Reuse FileDownloadButton for
  both the audio AND file rows — it decrypts before saving, adds loading/success
  state + a toast, and derives a filename extension.
- Audio playback bypassed MAudio's mimetype sanitization; pass the value through
  getBlobSafeMimeType (e.g. application/ogg → audio/ogg) so encrypted/odd-mimetype
  audio actually plays. Give voice messages a real filename+extension.

Also corrected the docs: AudioContent renders a seek bar, not an MSC3245 waveform.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

LOTUS_FEATURES.md

@@ -1084,7 +1084,7 @@ A toggle in **Settings → Privacy** switches between sending `m.read` (public r
 
 - Four tabs: **Images**, **Videos**, **Audio**, **Files** (each with a live count)
 - **Images/Videos** — a month-grouped grid; tiles decrypt on demand (lazy, near-viewport), open a keyboard-navigable **lightbox** (←/→/Esc, prev/next)
-- **Audio** — voice messages + audio files (`m.audio`) with an inline player (reuses `AudioContent`: play/seek/**speed control**, MSC3245 voice waveform; decrypts on play)
+- **Audio** — voice messages + audio files (`m.audio`) with an inline player (reuses `AudioContent`: play/seek/**speed control**; decrypts on play)
 - **Files** — name/size/sender rows with download
 - **Jump to message** — a "Go to message" action on file rows, audio rows, and in the lightbox navigates the timeline to the source event (`useRoomNavigate`) and closes the drawer
 - Encrypted media is decrypted client-side on demand (no lock placeholder); download works for all types