feat(chat): desktop-parity session resume — warm cache + bounded retry (#766)

Bring the desktop's session reconnection UX to mobile:

- Warm-cache fast-path: switchSession paints the cached Room transcript
  immediately instead of blanking the screen, then refreshes from the
  server in parallel (stale cache can't clobber a fresher server page)
- Bounded auto-retry: a failed session.resume (WS reject or REST
  transcript failure) retries with exponential backoff 1s->2s->4s->8s,
  capped at 4 attempts; WS + REST failures in the same cycle count as one
- Explicit error recovery: after exhaustion the chat shows an error card
  with a manual Retry instead of latching the spinner forever; manual
  retry resets the backoff cycle
- Reconnect-aware resume: gateway (re)connect clears stale error state
  and re-resumes the current session on the new socket
- Unit tests: warm-cache paint, bounded retry exhaustion, WS+REST
  dedupe, manual retry reset, reconnect restart

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/res/values/strings.xml

@@ -119,6 +119,7 @@
     <string name="chat_empty_title">Ready to chat</string>
     <string name="chat_empty_subtitle">Send a message to start a conversation</string>
     <string name="chat_status_connecting">Connecting to Hermes\u2026</string>
+    <string name="chat_resume_error_title">Couldn\'t connect to this session</string>
     <string name="chat_status_reconnecting">Reconnecting\u2026</string>
     <string name="chat_status_disconnected">Disconnected</string>
     <string name="chat_status_no_network">No network</string>