Add hash routing and Dota 2 logo to relations preview.

- New web/relations/router.js (parseHash/serializeHash/installRouter):
  top tabs, hero + detail sub-tab, item, patch version, tag filters and
  search query all sync to URL (#/heroes/axe/core, ?tags=...&q=...);
  pushState for discrete picks, replaceState for debounced search.
- export_relations_site.py copies router.js so static export deep-links.
- Header shows dota2_logo.png + title, served via /ui-icon/ whitelist.
- fetch_dota2_logo.py fetches the transparent emblem asset.
- Docs: CHANGELOG / README / DESIGN / AGENTS updated.

fetch_dota2_logo.py

@@ -0,0 +1,77 @@
+"""Fetch the official Dota 2 emblem icon (no wordmark) for the preview header.
+
+Source (discovered, not guessed) from the dota2.com.cn site HTML:
+  <link rel="Shortcut Icon" href="//www.dota2.com.cn/favicon.ico"/>
+
+That .ico is a multi-frame container whose largest frames are 256x256 PNGs of
+the red Dota 2 map-marker emblem with a transparent background — i.e. the
+brand icon alone, no "DOTA 2" text. We extract the first 256x256 PNG frame,
+decode with OpenCV, and write it to assets/ui_icons/dota2_logo.png so it rides
+the existing /ui-icon/ route + static-export glob pipeline.
+"""
+import struct
+import sys
+
+import cv2
+import numpy as np
+
+from common import UI_ICONS
+from http_utils import http_bytes
+
+FAVICON_URL = "https://www.dota2.com.cn/favicon.ico"
+OUT = UI_ICONS / "dota2_logo.png"
+TARGET_SIZE = 256
+
+
+def _extract_largest_png_frame(data: bytes) -> bytes | None:
+    """Return the bytes of the largest PNG-embedded frame in an ICO blob."""
+    if len(data) < 6 or data[:4] != b"\x00\x00\x01\x00":
+        return None
+    count = struct.unpack("<H", data[4:6])[0]
+    best: tuple[int, bytes] | None = None
+    for i in range(count):
+        off = 6 + 16 * i
+        if off + 16 > len(data):
+            break
+        w = data[off]
+        size = struct.unpack("<I", data[off + 8 : off + 12])[0]
+        img_off = struct.unpack("<I", data[off + 12 : off + 16])[0]
+        width = 256 if w == 0 else w
+        blob = data[img_off : img_off + size]
+        if blob[:8] != b"\x89PNG\r\n\x1a\n":
+            continue
+        if best is None or width * width > best[0]:
+            best = (width * width, blob)
+    return best[1] if best else None
+
+
+def main() -> int:
+    if OUT.is_file() and OUT.stat().st_size >= 512:
+        print(f"exists: {OUT} ({OUT.stat().st_size} bytes)")
+        return 0
+    data = http_bytes(FAVICON_URL, timeout=30)
+    if not data or len(data) < 16:
+        print("empty favicon download", file=sys.stderr)
+        return 1
+    png = _extract_largest_png_frame(data)
+    if png is None:
+        print("no PNG frame in .ico", file=sys.stderr)
+        return 1
+    arr = cv2.imdecode(np.frombuffer(png, dtype=np.uint8), cv2.IMREAD_UNCHANGED)
+    if arr is None:
+        print("decode failed", file=sys.stderr)
+        return 1
+    UI_ICONS.mkdir(parents=True, exist_ok=True)
+    ok = cv2.imwrite(str(OUT), arr)
+    if not ok:
+        print("write failed", file=sys.stderr)
+        return 1
+    h, w = arr.shape[:2]
+    channels = arr.shape[2] if arr.ndim == 3 else 1
+    transparent = int(np.count_nonzero(arr[:, :, 3] == 0)) if channels == 4 else 0
+    print(f"saved {OUT} ({w}x{h}, {OUT.stat().st_size} bytes, {transparent} px transparent)")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())