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_service.h

@@ -95,7 +95,7 @@ public:
     // quit), then resolveOutgoing() upserts the final status once the broadcast completes. A stray
     // persisted Sending (crash mid-broadcast) loads back as Sent.
     bool recordOutgoingPending(const ChatMessage& message);
-    void resolveOutgoing(const std::string& txid, ChatDelivery delivery);
+    void resolveOutgoing(const std::string& txid, ChatDelivery delivery, const std::string& reason = {});
 
     const ChatStore& store() const { return store_; }
     ChatStore& store() { return store_; }