Rework the Icon library to the wireframe

Section 16 of the audit page draws the three actions on the title row, a
magnifier inside the search box, caps section headings, and names its complaint
outright: "Ô icon trong lưới hiện tại không có viền/hover rõ khi rê chuột hay
khi đang chọn" — you could not tell which cell you were about to pick.

So: Thêm / Dán / Xoá move from the strip under both grids (where they read as
belonging to the custom grid alone) up beside the title; the search box gets its
magnifier; the two headings become caps through the same #navSectionHdr style
the rail uses; and #iconGrid cells take an accent border on hover and on
selection, the card language Agent and Công cụ already use.

Wording follows the drawing too: "ICON TÍCH HỢP" rather than "Icon có sẵn", and
"Tìm icon theo tên…" rather than "Tìm icon có sẵn…".

Left alone, and worth a decision: the drawing labels the buttons "+ Thêm icon"
and "Xoá", where the app says "Thêm tệp SVG" and "Xóa tùy chỉnh". The longer
labels say which file type is wanted and that only custom icons can be deleted,
so shortening them to match the drawing would cost more than it gains.

check_icons_screen covers the four points. Its first version passed a mutation
that dropped the buttons from the header layout — unparented they sit at (0,0),
which read as "the same row" — so it now requires the vertical centres to line
up AND each button to start right of the title.

20/20 checkers pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

i18n.py

@@ -2472,8 +2472,8 @@ STRINGS: Dict[str, Dict[str, str]] = {
               "下から独自のSVGアイコンを追加でき、名前ですぐ使えます。",
         "vi": "Các icon dùng cho agent và flow. Gõ tên vào ô Icon của step/agent để dùng. Thêm icon SVG "
               "của bạn ở dưới — dùng được ngay bằng tên."},
-    "icons_admin.search": {"en": "Search built-in icons…", "ja": "組込みアイコンを検索…", "vi": "Tìm icon có sẵn…"},
-    "icons_admin.builtin": {"en": "Built-in icons", "ja": "組込みアイコン", "vi": "Icon có sẵn"},
+    "icons_admin.search": {"en": "Search icons by name…", "ja": "名前でアイコンを検索…", "vi": "Tìm icon theo tên…"},
+    "icons_admin.builtin": {"en": "Built-in icons", "ja": "組込みアイコン", "vi": "Icon tích hợp"},
     "icons_admin.custom": {"en": "Custom icons", "ja": "カスタムアイコン", "vi": "Icon tùy chỉnh"},
     "icons_admin.add": {"en": "Add SVG file", "ja": "SVGファイルを追加", "vi": "Thêm tệp SVG"},
     "icons_admin.paste": {"en": "Paste SVG", "ja": "SVGを貼付", "vi": "Dán SVG"},