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/chat/chat_database.h

@@ -43,6 +43,11 @@ public:
     // Returns true if newly inserted; false on duplicate or while locked.
     bool append(const ChatMessage& message);
 
+    // Persist-or-overwrite one message by its (txid+position) dedup key. Unlike append(), this
+    // updates an existing row's payload — used for outgoing echoes whose delivery status changes
+    // (Sending → Sent/Failed). Returns true on success; false while locked / on error.
+    bool upsert(const ChatMessage& message);
+
     // Decrypt and return every stored message for the unlocked wallet, in insertion order. Empty
     // while locked or if none. Rows that fail to decrypt/parse are skipped.
     std::vector<ChatMessage> load();