fix(chat): address adversarial review of the send-path change

Four confirmed findings from the review of ef247c9:

1. Persistence regression — the deferred-persist echo (in-memory Sending, written
   only when the async callback resolved) meant a message broadcast on-chain but
   whose callback hadn't fired yet was LOST from history if the app quit/crashed
   in that window. Persist the echo immediately as Sending and UPSERT the final
   status on resolve (new ChatDatabase::upsert with ON CONFLICT DO UPDATE, since
   append is INSERT-OR-IGNORE). A stray persisted Sending still loads as Sent.

2. Fee ceiling — dragonxd REJECTS a 0-value tx whose fee exceeds the default
   miners fee (0.0001), and max(getDefaultFee(), 0.0001) can only raise it, so a
   default_fee > 0.0001 broke every chat send. Pin chat to exactly kChatMinFeeDrgx,
   dropping getDefaultFee() from this path (chat always moves 0 value).

3. Lifetime — the resolve callback had no generation guard, so a wallet lock (which
   doesn't disconnect) between submit and callback could resolve against a cleared
   store. Capture chat_session_generation_ and bail on mismatch (both the full-node
   callback and the lite optimistic resolve), matching the identity-fetch pattern.

4. Retry misdirect — Retry on a failed CONTACT REQUEST called sendChatMessage,
   which (no peer key yet) just showed "waiting for reply". Route it to
   sendContactRequestForCid() (refactored out of startChatConversation) so it
   re-sends the request into the SAME conversation.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.h

@@ -179,6 +179,9 @@ public:
     // message (to a conversation whose peer key we know) / a new-conversation contact request.
     void sendChatMessage(const std::string& conversationId, const std::string& text);
     void startChatConversation(const std::string& peerZaddr, const std::string& text);
+    // Send a contact request into an existing conversation (used to retry a failed request in place).
+    void sendContactRequestForCid(const std::string& cid, const std::string& peerZaddr,
+                                  const std::string& text);
     // Debug/sweep convenience: give the Chat tab a demo identity + a few sample conversations
     // (in-memory only, not persisted) so the screenshot sweep captures the populated UI. No-op when
     // the chat feature is off.