feat(chat): enable HushChat by default + legacy (non-mnemonic) wallet support

Flip DRAGONX_ENABLE_CHAT to default ON (a fresh configure now builds chat in),
and make full-node chat work for every wallet + any daemon.

Legacy compat: chat identity is derived from the wallet's mnemonic
(z_exportmnemonic) for a portable, SDXLite-compatible identity — but a legacy
random-seed wallet has no mnemonic, and released daemons don't have that RPC at
all. Provisioning now falls back to a stable z-address's spending key
(z_exportkey) in those cases: a functional, wallet-local identity that works on
any daemon. Since a chat identity is local (peers learn your public key from
your message headers, not your derivation), interop is unaffected; only
cross-client portability needs the mnemonic. The spending key is an in-memory
KDF input over a key the wallet already holds, wiped after use — no new exposure.

Stability: the chosen chat z-address (the reply-to in headers AND the legacy
identity source) is now persisted in settings (chat_reply_zaddr), so the
identity + reply address don't shift when new addresses are generated.
chatReplyZaddr() picks the smallest spendable z-addr once and reuses it.

CLAUDE.md updated to reflect the default flip. Verified: Linux + Windows build
with chat ON, fresh-configure default confirmed ON, ctest 100%, hygiene clean.

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

src/config/settings.h

@@ -108,6 +108,12 @@ public:
     std::string getSkinId() const { return skin_id_; }
     void setSkinId(const std::string& id) { skin_id_ = id; }
 
+    // Stable z-address chosen for HushChat: the reply-to address in outgoing headers, and (for
+    // legacy/non-mnemonic wallets) the source of the seed-derived chat identity. Persisted so the
+    // identity + reply address don't shift when new addresses are generated.
+    std::string getChatReplyZaddr() const { return chat_reply_zaddr_; }
+    void setChatReplyZaddr(const std::string& z) { chat_reply_zaddr_ = z; }
+
     // Privacy
     bool getSaveZtxs() const { return save_ztxs_; }
     void setSaveZtxs(bool save) { save_ztxs_ = save; }
@@ -410,6 +416,7 @@ private:
     // Settings values
     std::string theme_ = "dragonx";
     std::string skin_id_ = "dragonx";
+    std::string chat_reply_zaddr_;
     bool save_ztxs_ = true;
     bool auto_shield_ = true;
     bool use_tor_ = false;