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

src/chat/chat_service.cpp

@@ -189,8 +189,8 @@ bool ChatService::recordOutgoingPending(const ChatMessage& message) {
     return appended;
 }
 
-void ChatService::resolveOutgoing(const std::string& txid, ChatDelivery delivery) {
-    const ChatMessage* updated = store_.updateDelivery(txid, delivery);
+void ChatService::resolveOutgoing(const std::string& txid, ChatDelivery delivery, const std::string& reason) {
+    const ChatMessage* updated = store_.updateDelivery(txid, delivery, reason);
     if (updated && db_) db_->upsert(*updated);  // overwrite the Sending row with the final status
 }
 

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_; }

src/chat/chat_store.cpp

@@ -61,10 +61,12 @@ std::vector<ChatMessage> ChatStore::conversation(const std::string& conversation
     return out;
 }
 
-const ChatMessage* ChatStore::updateDelivery(const std::string& txid, ChatDelivery delivery) {
+const ChatMessage* ChatStore::updateDelivery(const std::string& txid, ChatDelivery delivery,
+                                             const std::string& reason) {
     for (auto& message : messages_) {
         if (message.txid == txid) {
             message.delivery = delivery;
+            message.fail_reason = (delivery == ChatDelivery::Failed) ? reason : std::string();
             ++revision_;
             return &message;
         }

src/chat/chat_store.h

@@ -40,7 +40,8 @@ public:
 
     // Update an outgoing echo's delivery status by its local txid. Returns a pointer to the updated
     // message (for the caller to persist), or nullptr if no message has that txid.
-    const ChatMessage* updateDelivery(const std::string& txid, ChatDelivery delivery);
+    const ChatMessage* updateDelivery(const std::string& txid, ChatDelivery delivery,
+                                      const std::string& reason = {});
 
     // Distinct conversation ids, in first-seen order.
     std::vector<std::string> conversationIds() const;