feat(ux): clearer chat delivery, explorer empty/range states, honest settings label

Three next-batch UX-audit fixes:
- Chat delivery: the sending/sent glyph has higher contrast and a tooltip
  ("Sent to the network — arrives after it confirms", so a check isn't mistaken
  for "delivered"); a failed send now carries the real reason (ChatMessage::
  fail_reason, set from onChatBroadcastResult via friendlyRpcError, in-memory
  only) in the "not sent" hover tooltip.
- Explorer: a block height beyond the tip says so ("Block N is beyond the chain
  tip (M)") instead of silently snapping to the top; the empty block list shows
  Not connected / Syncing (N behind) / Loading instead of an endless "Loading".
- Settings: the misleading "Reset to Defaults" button (which only reloads the
  last-saved values) is relabeled "Discard unsaved changes" with accurate copy.

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

src/chat/chat_message.h

@@ -26,7 +26,9 @@ struct ChatMessage {
     std::string body;                 // decrypted plaintext (Message) or request text (ContactRequest)
     std::int64_t timestamp = 0;       // tx time in seconds; set by the ingesting caller
     std::size_t payload_position = 0; // together with txid, the dedup key
-    ChatDelivery delivery = ChatDelivery::Sent;  // outgoing only
+    ChatDelivery delivery = ChatDelivery::Sent;  // outgoing only. Sent = broadcast to the network (not
+                                                 // necessarily confirmed/delivered yet).
+    std::string fail_reason;                     // outgoing Failed only; in-memory, NOT persisted
 };
 
 } // namespace dragonx::chat