feat(dashboard): the console, phase 1 of the redesign (preview at /static/console.html)

The user chose direction B of the redesign: a full-screen operations console.
It is built next to the current dashboard, served by ServeDir at
/static/console.html (noindex, links back to /), and replaces dashboard.html
once it covers every feature (docs/DASHBOARD_REDESIGN.md, phase 4).

Phase 1:
- top bar: the Chiang Mai verdict (P.1 level + 24 h forecast, the current
  dashboard's rule), gauges reporting, highest 24 h flood chance, data age
  ("overnight pause" instead of "stale" during RID's nightly gap, as in the
  notifier), EN/TH, light/dark (dark default), Get alerts
- gauge list: one status rule for list, map and panel (normal, watch within
  0.5 m or >= 25 % chance, warning, danger, no data), margin to the warning
  level, 72 h sparkline, 6 h change; filter; sort by river order, closest to
  the warning level, rising; arrow keys
- map: Ping mainstem and tributaries, status markers with code + level,
  layers Rain 24 h (on by default only with heavy rain) and Flood zones
  (auto-shown while P.1 is at a stage), one legend
- station panel: level, change, discharge (last RID value when the newest
  hour is level-only), warning/danger levels with margins, capacity, level
  scale with now and 24 h peak, 72 h chart with the forecast peaks, forecast
  table with the model and issue time, the P.1 inundation stages, data links
- alerts drawer with the ntfy topics; selection in the URL (#P.84); refresh
  every 5 minutes
- Inter and Noto Sans Thai self-hosted (src/static/fonts, 75 KB, OFL), no
  emoji, Thai first, Bangkok time

tests/test_console.py: routed fetch paths, EN/TH parity, translated aria
labels, no emoji, fonts and licences, noindex. dev_proxy.py serves
src/static with real content types (it sent HTML as octet-stream).

scripts/dev_proxy.py

@@ -1,8 +1,12 @@
 """Serve the working-copy dashboard locally with API calls proxied to the
 live server, so browser-side changes can be checked against real data
-before deploy. Usage: python scripts/dev_proxy.py [port]"""
+before deploy. Usage: python scripts/dev_proxy.py [port]
+
+/ is src/static/dashboard.html, /static/... any file in src/static (e.g. the
+console preview at /static/console.html), everything else the live server."""
 
 import http.server
+import mimetypes
 import os
 import sys
 import urllib.request
@@ -30,8 +34,10 @@ class Handler(http.server.BaseHTTPRequestHandler):
                 ctype = (
                     "application/json"
                     if f.suffix in (".json", ".geojson")
-                    else "application/octet-stream"
+                    else mimetypes.guess_type(f.name)[0] or "application/octet-stream"
                 )
+                if ctype.startswith("text/"):
+                    ctype += "; charset=utf-8"
                 self._send(200, ctype, f.read_bytes())
                 return
         try: