feat(dashboard): CMU's 2024 flood marks as a map layer

CMU Water Center marked the October 2024 high-water line at ~4,900 places in
Chiang Mai and Lamphun (depth above the road while P.1 stood at 5.30 m).

- src/static/flood-marks-2024.json (position and depth, 110 KB), built by
  scripts/build_flood_marks.py from CMU's GeoJSON.
- "Flood marks 2024" layer: canvas dots from zoom 12 in CMU's five depth
  classes, legend row, CMU credit; turning it on below zoom 12 flies to the
  city. A mark's popup gives the depth and its flood zone.
- Zone and marker popups name the nearest mark within 250 m (loaded with the
  zones).
- No depth estimate for other P.1 levels: the marks do not scale one-to-one
  with P.1 (marks in the 5.30 m zone stood a median 65 cm deep in 2024).
- The layer bar wraps on narrow maps; popups pan clear of it.
- Docs: DATA_SOURCES 5.1, DASHBOARD_REDESIGN, README.

scripts/build_flood_marks.py

@@ -0,0 +1,71 @@
+"""Build src/static/flood-marks-2024.json from CMU Water Center's flood marks.
+
+After the October 2024 flood (P.1 peaked at 5.30 m on 5 October), CMU Water
+Center marked the high-water line at about 5,000 places in Chiang Mai and
+Lamphun (CMFlood, "Flood marks 2024"): the depth above the road surface, in
+cm. This keeps position and depth only, in a compact array form:
+
+    uv run python scripts/build_flood_marks.py              # fetch from CMU
+    uv run python scripts/build_flood_marks.py --geojson F  # or a saved copy
+"""
+
+import argparse
+import json
+import urllib.request
+from pathlib import Path
+
+REPO = Path(__file__).resolve().parents[1]
+OUT = REPO / "src" / "static" / "flood-marks-2024.json"
+URL = "https://watercenter.cmu.ac.th/data/New_floodMark_2025_fixed_like_pole.geojson"
+SOURCE = (
+    "CMU Water Center (ศูนย์วิชาการสนับสนุนการบริหารจัดการน้ำ มหาวิทยาลัยเชียงใหม่), "
+    "flood marks of October 2024, "
+    "https://watercenter.cmu.ac.th/chiangmai/cmflood/warning/floodmark"
+)
+
+
+def load(path: Path | None) -> dict:
+    if path:
+        return json.loads(path.read_text(encoding="utf-8"))
+    req = urllib.request.Request(URL, headers={"User-Agent": "Mozilla/5.0"})
+    with urllib.request.urlopen(req, timeout=120) as r:
+        return json.loads(r.read().decode("utf-8"))
+
+
+def main() -> None:
+    ap = argparse.ArgumentParser(description=__doc__.splitlines()[0])
+    ap.add_argument("--geojson", type=Path, help="read CMU's GeoJSON from here")
+    args = ap.parse_args()
+
+    marks, skipped = [], 0
+    for f in load(args.geojson)["features"]:
+        p = f.get("properties") or {}
+        lon, lat, depth = p.get("longitude"), p.get("latitude"), p.get("water_level")
+        if lon is None or lat is None or depth is None:
+            skipped += 1
+            continue
+        # 18-19.5 N, 98.5-99.5 E: a swapped or empty coordinate is skipped
+        if not (18.0 < lat < 19.5 and 98.5 < lon < 99.5) or depth <= 0:
+            skipped += 1
+            continue
+        marks.append([round(lon, 5), round(lat, 5), round(depth)])
+    marks.sort(key=lambda m: (m[1], m[0]))
+    doc = {
+        "source": SOURCE,
+        "p1_level": 5.30,
+        "date": "2024-10-05",
+        "fields": ["lon", "lat", "depth_cm_above_road"],
+        "marks": marks,
+    }
+    OUT.write_text(
+        json.dumps(doc, ensure_ascii=False, separators=(",", ":")) + "\n",
+        encoding="utf-8",
+    )
+    print(
+        f"wrote {OUT.relative_to(REPO)}: {len(marks)} marks, {skipped} skipped, "
+        f"{OUT.stat().st_size / 1024:.0f} KB"
+    )
+
+
+if __name__ == "__main__":
+    main()