feat(chat): emoji picker as an in-pane overlay with keyword search

Reworked the emoji picker from a floating popup (which overlapped the message
area) into an overlay that takes over the conversation-list pane while open:
a Cancel button + a keyword search box at the top, then a responsive grid below
that wraps to the pane width.

Each emoji now carries search keywords (grin/heart/fire/…) so the search box
filters the ~150-emoji set. The 🙂 composer button toggles the overlay;
selecting an emoji appends it to the composer (byte-cap respected) and the
overlay stays open for multiple picks.

+1 CJK glyph (絵) for the ja "Search emoji" string.

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

res/lang/ja.json

@@ -139,6 +139,7 @@
     "chat_cancel": "キャンセル",
     "chat_contact_added": "連絡先を追加しました — 連絡先で名前を変更できます",
     "chat_contact_request": "連絡リクエスト",
+    "chat_emoji_search": "絵文字を検索",
     "chat_empty_hint": "まだ会話はありません。受信したメッセージはここに表示されます。",
     "chat_empty_start": "「新しい会話」から始めましょう。",
     "chat_empty_title": "会話はまだありません",