Add end-to-end encrypted voice messages; release v5.4.5

- Record voice notes in-browser, sent over the chunked AES-GCM file-transfer
  channel (per-file session key + signed SHA-256 integrity).
- Captured as PCM and encoded to WAV for universal playback (incl. iOS/Safari);
  auto-accepted and played inline from an in-memory blob, never written to disk.
- Composer mic button with live waveform + timer; desktop shows mic + send side
  by side, mobile swaps mic to send when typing.
- CSP media-src now allows blob: so recorded/received audio can play.
- Roadmap: Desktop Edition -> 5.0, new 5.5 'Secure Voice & Calls', later
  milestones shifted; version bumped to 5.4.5.
- Update README, docs (security/API/cryptography), and CHANGELOG.

.dockerignore

@@ -9,3 +9,8 @@ fly.toml
 *.log
 .DS_Store
 **/.DS_Store
+# Source maps are debug-only and never needed at runtime; excluding them keeps the
+# build context small (the .map files are several MB) and the deploy upload fast.
+**/*.map
+# Marketing screenshots are referenced only from the README, not served by the app.
+assets/screenshots