feat(chat): emoji picker + show-hidden toggle

- Emoji picker: a 🙂 button on the composer row opens a scrollable grid of ~150
  common single-codepoint emoji (all verified present in the bundled NotoEmoji
  subset); clicking appends to the composer, respecting the byte cap. ImGui does
  no shaping, so the set is single-codepoint only (ZWJ sequences / flags omitted).
- Show hidden: when there are hidden conversations, a "Show hidden (N)" toggle
  appears in the list; toggling it reveals them (dimmed) and the thread header's
  Hide button becomes Unhide. Off by default, and resets when nothing is hidden.

8-language strings; no new CJK glyphs.

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

res/lang/zh.json

@@ -148,6 +148,7 @@
     "chat_export_warn": "将解密后的消息保存为纯文本。请妥善保管该文件。",
     "chat_hidden_toast": "会话已隐藏——收到新消息后会重新显示",
     "chat_hide": "隐藏",
+    "chat_hide_hidden": "收起已隐藏",
     "chat_jump_latest": "最新",
     "chat_len_over": "消息过长",
     "chat_locked_hint": "解锁钱包以加载您的聊天记录。",
@@ -167,6 +168,7 @@
     "chat_send": "发送",
     "chat_send_failed": "未发送",
     "chat_sending": "发送中…",
+    "chat_show_hidden": "显示已隐藏",
     "chat_time_now": "刚刚",
     "chat_toast_compose_failed": "无法编写该消息（内容过长？）。",
     "chat_toast_lite_busy": "已有发送正在进行中，或未打开任何钱包。",
@@ -176,6 +178,7 @@
     "chat_toast_request_compose_failed": "无法编写联系人请求（地址或文本无效？）。",
     "chat_toast_request_queued": "联系人请求已排队。",
     "chat_toast_waiting_reply": "等待该联系人回复——对方回复后您即可向其发送消息。",
+    "chat_unhide": "取消隐藏",
     "chat_unmute": "取消静音",
     "chat_waiting_reply": "等待该联系人回复——对方回复后您即可向其发送消息。",
     "chat_you": "我",