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.

README.md

@@ -38,10 +38,10 @@ background in [Teaching a Model to See the Ping River Rise 13 Hours Early](https
   3.70 m. Everything about the model, including what did not work:
   [docs/FLOOD_FORECASTING.md](docs/FLOOD_FORECASTING.md).
 - **Shows it.** One screen: Chiang Mai's flood verdict, the gauges with their status
-  and trend, a self-hosted map of the basin with rain and the city's inundation
-  zones, and a panel per station with its history since 2018, the forecast and how
-  accurate it has been. ThaiWater's basin stations, a replay of the 2024 flood,
-  Thai/English, dark/light.
+  and trend, a self-hosted map of the basin with rain, the city's flood zones and
+  the high-water marks of 2024, and a panel per station with its history since
+  2018, the forecast and how accurate it has been. ThaiWater's basin stations, a
+  replay of the 2024 flood, Thai/English, dark/light.
 - **Notifies.** Public push alerts over a self-hosted [ntfy](https://ntfy.sh): one
   message when a gauge crosses its warning or danger level, one all-clear on the
   way down, an opt-in early-warning topic from the model, nothing in between.