[0.9.1] FORK_IA_UX v2: fix top-bar (visibility sheet) + Karvan module (ephemeral messages + WebRTC)

Navigation fixes:
- The bottom "Explore" sheet no longer blocks the top bar: closed state is
  visibility:hidden (out of hit-testing), so the top identity buttons and page
  content always receive taps (fixes "top buttons don't work").
- Top bar is now identity only (logo, avatar). The Personal/Community filter
  moved into the Explore sheet, next to the hive, as a segmented control.

New module "Karvan" (self-contained; inspired by karvan-protocol ephemeral rooms):
- Ephemeral/temporary chat rooms held only in RAM, self-destructing on idle
  (30 min) / absolute (2 h) TTL — nothing is written to disk.
- Server relay chat (RAM + polling) as the reliable path, plus a WebRTC
  data-channel layer (perfect negotiation, HTTP signaling mailbox) for instant
  P2P delivery; degrades to the relay if WebRTC is unavailable. Data-channel
  only (no camera/mic) so no wrapper permission changes are needed.
- Files: models/karvan_model.js, views/karvan_view.js, client/public/js/karvan.js;
  routes /karvan* + karvanMod config + nav entry (network) + karvanTitle i18n (11 langs).

Additive and gated by OASIS_MOBILE / karvanMod; Linux and the UX_OASIS branch untouched.

src/configs/oasis-config.json

@@ -34,6 +34,7 @@
     "reportsMod": "on",
     "opinionsMod": "on",
     "padsMod": "on",
+    "karvanMod": "on",
     "calendarsMod": "on",
     "transfersMod": "off",
     "feedMod": "on",