feat: ski runs layer in map-external-layer sample

[lpernelle-woosmap]

Summary

• Replace ArcGIS satellite tiles with OpenSnowMap ski runs as the default third-party layer in the map-external-layer sample
• Default style set to Light Grey, initial view centered on Oz-en-Oisans (zoom 15.64)
• OpenWeather layers (temperature, precipitation) now fly to a US view when selected
• Layer change updates the map bounds via flyTo; opacity change no longer recreates the map

Code quality

While in there, fixed several issues in the existing sample code:

• Bug — overlay stacking: changeLayer used insertAt(0, …) without clearing previous overlays. After N changes, N raster layers were stacked with cumulative opacity. Now applyLayer calls overlayMapTypes.clear() first.
• Bug — opacity validation: parseFloat(opacityInput.value) was called twice and NaN (empty/non-numeric input) silently passed validation. Centralized in readOpacity() with a proper isNaN check.
• Bug — opacity change reset bounds: the original single change handler called changeStyle on every event, recreating the map even on opacity changes. Split into onStyleChange / onLayerChange / onOpacityChange.
• Refactor: replaced if/else chains for style and layer selection with Record<string, …> lookups (STYLE_PRESETS, LAYER_CONFIGS, LAYER_VIEWS). Reduces cognitive complexity and makes adding a new layer/style a one-line change.
• TypeScript: split combined let styleSelect, layerSelect: HTMLSelectElement (where only layerSelect was actually typed), String → string, == → ===, consistent indentation, semicolons.

Follow-up (out of scope)

• The OpenWeather API key is still hardcoded in samples/map-external-layer/index.ts:1 (pre-existing on master). Should be rotated and replaced with a placeholder.
• dist/samples/map-external-layer/** is intentionally not included in this PR — CI regenerates it via the chore: update dist folder [skip ci] job.

Test plan

• Load the sample — map renders centered on Oz-en-Oisans with Light Grey style and OpenSnowMap ski runs overlay
• Switch layer to OpenWeather temperature → map flies to US, overlay updates with no flicker / no stacking
• Switch back to ski runs → flies back to Oz, overlay swaps
• Change opacity slider — overlay opacity updates without recreating the map / changing center
• Switch style (Default / Light Grey / Night / Retro) — base map style updates and current layer is reapplied

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[shwetadeshmukh]

👍🏼