feat(chat): hide conversations + contact address picker for new conversation

Hide conversations:
- A "Hide" action in the thread header drops a conversation from the list. The
  messages stay in the seed-encrypted store (on-chain history can't be deleted);
  a new INCOMING message un-hides it (you can't un-receive), so nothing is lost.
- Hidden cids persist in settings (mirrors the mute list) and are skipped by both
  the conversation list and the unread badge.

New-conversation address picker:
- A "Choose from contacts" dropdown lists the address book's shielded (z-address)
  contacts and fills the recipient field on selection; manual paste still works.

8-language strings + CJK subset (+1 glyph 届).

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

src/config/settings.h

@@ -130,6 +130,19 @@ public:
                                    muted_chat_cids_.end());
     }
 
+    // Hidden chat conversations (by cid) — hidden ones are filtered out of the list; a new incoming
+    // message un-hides them (you can't un-receive) so nothing is silently lost.
+    bool isChatHidden(const std::string& cid) const {
+        return std::find(hidden_chat_cids_.begin(), hidden_chat_cids_.end(), cid) != hidden_chat_cids_.end();
+    }
+    void setChatHidden(const std::string& cid, bool hidden) {
+        const bool already = isChatHidden(cid);
+        if (hidden && !already) hidden_chat_cids_.push_back(cid);
+        else if (!hidden && already)
+            hidden_chat_cids_.erase(std::remove(hidden_chat_cids_.begin(), hidden_chat_cids_.end(), cid),
+                                    hidden_chat_cids_.end());
+    }
+
     // Privacy
     bool getSaveZtxs() const { return save_ztxs_; }
     void setSaveZtxs(bool save) { save_ztxs_ = save; }
@@ -486,6 +499,7 @@ private:
     std::string skin_id_ = "dragonx";
     std::string chat_reply_zaddr_;
     std::vector<std::string> muted_chat_cids_;   // muted chat conversations by cid (Q10)
+    std::vector<std::string> hidden_chat_cids_;  // hidden chat conversations by cid
     bool save_ztxs_ = true;
     bool auto_shield_ = true;
     bool use_tor_ = false;