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/ko.json

@@ -139,6 +139,7 @@
     "chat_cancel": "취소",
     "chat_contact_added": "연락처 추가됨 — 연락처에서 이름을 변경하세요",
     "chat_contact_request": "연락 요청",
+    "chat_emoji_search": "이모지 검색",
     "chat_empty_hint": "아직 대화가 없습니다. 받은 메시지가 여기에 표시됩니다.",
     "chat_empty_start": "\"새 대화\"로 시작하세요.",
     "chat_empty_title": "아직 대화가 없습니다",