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.cpp

@@ -152,6 +152,11 @@ bool Settings::load(const std::string& path)
             for (const auto& c : j["muted_chat_cids"])
                 if (c.is_string()) muted_chat_cids_.push_back(c.get<std::string>());
         }
+        if (j.contains("hidden_chat_cids") && j["hidden_chat_cids"].is_array()) {
+            hidden_chat_cids_.clear();
+            for (const auto& c : j["hidden_chat_cids"])
+                if (c.is_string()) hidden_chat_cids_.push_back(c.get<std::string>());
+        }
         loadScalar(j, "acrylic_enabled", acrylic_enabled_);
         loadScalar(j, "acrylic_quality", acrylic_quality_);
         loadScalar(j, "blur_multiplier", blur_multiplier_);
@@ -430,6 +435,9 @@ bool Settings::save(const std::string& path)
     j["muted_chat_cids"] = json::array();
     for (const auto& c : muted_chat_cids_)
         j["muted_chat_cids"].push_back(c);
+    j["hidden_chat_cids"] = json::array();
+    for (const auto& c : hidden_chat_cids_)
+        j["hidden_chat_cids"].push_back(c);
     j["acrylic_enabled"] = acrylic_enabled_;
     j["acrylic_quality"] = acrylic_quality_;
     j["blur_multiplier"] = blur_multiplier_;