Fix/basemap picker availability

[TommySquared]

Map shell fixes: basemap picker, theme pinning, draw layering, freehand

Base: main · 4 commits · server/mapcontrol_server/main.py (+108 / −10)

A batch of fixes to the served map shell surfaced while working through the EOGPT
map drawer, plus a new freehand draw mode.

What changed

Hide unavailable basemaps from the picker (9f745ff)
Config-load only drops basemap entries whose provider key env var is unset — a
set-but-invalid/plan-limited key still listed every style with blank previews that
broke on click. The picker now probes each entry's preview tile client-side and
removes failing tiles (pruning emptied group sections, hiding the control if <2
remain). The current basemap is never removed.

Freehand polygon draw mode (7327306)
Adds Terra Draw's TerraDrawFreehandMode as a fifth toolbar tool. Finished shapes
are Polygon geometry and flow through the existing pipeline as draw_type: 'polygon', so no server/SDK/consumer changes are needed; enable_drawing
accepts 'freehand'/'freedraw'. Included in the theme-flip restyle observer.

Pin ?theme= against snapshot replay; keep the globe surround dark (83c2368)

• An explicit ?theme= from the embedding page now wins over the stored map-level
  theme on session_snapshot restore (same rule as ?basemap=). Previously every
  WS (re)connect replayed the persisted/auto theme over the boot param, flipping
  a light-booted page's controls dark (or resolving auto to the OS preference
  regardless of the embedder). Live set_theme broadcasts still apply.
• The canvas surround behind the globe is now a fixed deep navy in both themes
  instead of --eo-bg, which rendered a white void behind the planet in light mode.

Picker hover, sketch layering, draw cursor (8624b28)

• MapLibre's stock .maplibregl-ctrl button:hover (translucent) outranked the
  picker's themed hover on specificity, blanking the trigger over the map on hover.
  Re-asserted the themed surfaces (trigger, tiles, draw-toolbar) at higher
  specificity — hover is now just the border/surface highlight.
• Keep the live Terra Draw sketch (td-*) and dashed pending previews above every
  asset layer via raiseDrawLayers(), called after each asset add, move_layer,
  and reorder_assets — they could otherwise be buried by new overlays/reorders.
• Crosshair cursor while a draw mode is armed (same class-on-canvas pattern as
  delete mode); the grab cursor read as "pan."

Testing

• python -c "import mapcontrol_server.main" imports clean (f-string HTML intact).
• Exercised the picker, freehand draw, theme flips, and draw/reorder layering
  against the local stack via the dev-mc2 compose override.

Notes for reviewers

• The theme-pinning fix is the mc2 half of the eogpt "map ignores theme flips"
  bug; the eogpt half (forwarding set_theme on theme change) ships in that repo.
• All changes are in the served-shell HTML/JS string in main.py; there are no
  Python API or model changes.