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.

docs/DASHBOARD_REDESIGN.md

@@ -51,7 +51,7 @@ One screen, no page scroll on a desktop:
 |---|---|
 | Top bar | brand · **Chiang Mai verdict** (P.1 level + 24 h forecast, always visible, click selects P.1) · gauges reporting · highest 24 h flood chance · data age · EN/ไทย · light/dark · Get alerts |
 | Left | two lists: **RID gauges** (status bar, code, name, margin to the warning level, 72 h sparkline, level and 6 h change; sort by river order, closest to the warning level, rising) and **Basin stations** (ThaiWater/HII: % of bank height, river and agency; sort fullest first or by name); filter; legend |
-| Centre | map on a self-hosted vector basemap (labels in the page language): Ping mainstem and tributaries, gauges coloured by status with code + level labels, layers Basin stations (diamonds), Rain 24 h and Flood zones, the team's markers from zoom 12, one legend |
+| Centre | map on a self-hosted vector basemap (labels in the page language): Ping mainstem and tributaries, gauges coloured by status with code + level labels, layers Basin stations (diamonds), Rain 24 h, Flood zones and Flood marks 2024, the team's markers from zoom 12, one legend |
 | Right | selected RID gauge: level, 6 h change, discharge, warning/danger level with margins, capacity; level scale with now and 24 h peak; history (3 d / 7 d / 30 d / 90 d / 1 y / all or dates: level, discharge, the 24 h peaks as issued, warning/danger bands, hover values; up to 7 d it ends in the current forecast); forecast table (peak, chance of warning/danger at 6/12/24 h, model, issue time, disclaimer); for P.1 the five city flood stages, the zones toggle and "How accurate is the forecast?"; data links. A basin station shows level (m MSL), % of bank height with ThaiWater's class, distance to and height of the bank top, gauge reading, agency (latest reading only: no history is kept for them) |
 | Status bar | data credits, "2.7M readings since 2018" (opens the data statistics), API, source code |
 
@@ -115,6 +115,7 @@ One screen, no page scroll on a desktop:
 - **Forecast accuracy:** loaded when the section is opened (a cold `/api/forecast/skill` query takes up to 20 s), cached for 15 minutes; bias and persistence error are in each row's tooltip.
 - **Replay of October 2024:** `src/static/flood-2024-all.json` (hourly readings of the 16 gauges, 15 September – 12 October 2024, and the 24 h warning chance and peak of a model trained on data before 1 September 2024). The page's clock, verdict, list, markers, flood zones and panel follow the replay; the panel's chart shows the season on fixed axes up to the replay hour. It plays hour by hour while the model alerts or P.1 is above 3.50 m, faster through quiet days, and can be paused and dragged. Rain, the basin stations and the 5-minute refresh are off during the replay, since they are today's data. A violet bar (REPLAY, date, P.1 level, the model's message) and the top bar's "Replay time" replace the live data age.
 - **Simulation:** `?demo_level=4.4` pins P.1 at 4.40 m; `?demo_rise=1` raises it by 5 cm every 0.7 s from the real level to the 2024 record (5.30 m). An amber SIMULATION bar names the real level; the top bar and the P.1 panel say "simulated". Only P.1 is simulated: the forecasts and other gauges stay live.
+- **Flood marks 2024:** `src/static/flood-marks-2024.json`, CMU Water Center's ~4,900 marks of the October 2024 high-water line (depth above the road at P.1 5.30 m; `scripts/build_flood_marks.py`), on a canvas from zoom 12 in CMU's five depth classes; turning the layer 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 (the marks load with the zones). Off by default; also available in the replay, since it is 2024 data. Popups pan clear of the layer bar, which wraps to two rows on phones.
 - **Basemap:** `src/static/basemap/ping-basin.pmtiles`, a Protomaps extract (bbox 97.85–99.95 E, 16.85–20.2 N, zoom 0–12, 15 MB, OpenStreetMap data), drawn by protomaps-leaflet 5.1.0 in its `light`/`dark` style with `name:th`/`name:en` labels. Refresh instructions in `src/static/basemap/README.md`. The map cannot be panned far outside the basin. OSM raster tiles are the fallback if the renderer does not load.
 
 ## Phases