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/util/i18n.cpp

@@ -252,6 +252,10 @@ void I18n::loadBuiltinEnglish()
         strings_["chat_len_over"] = "Message too long";
         strings_["chat_mute"] = "Mute";
         strings_["chat_unmute"] = "Unmute";
+        strings_["chat_hide"] = "Hide";
+        strings_["chat_hidden_toast"] = "Conversation hidden — a new message brings it back";
+        strings_["chat_pick_contact"] = "Choose from contacts\xE2\x80\xA6";
+        strings_["chat_no_z_contacts"] = "No shielded-address contacts yet";
         // Seed-phrase backup (full-node)
         strings_["seed_backup_button"] = "Seed phrase";
         strings_["tt_seed_backup"] = "Show and back up your wallet's 24-word recovery seed phrase";