feat: shareable URL for legend filter state (#531)

[fatherlinux]

Summary

• Legend filter state (POI types, boundaries, map layers) is now encoded in URL query parameters
• Users can copy the URL and share it — recipients see the same legend filters applied
• URL updates in real-time as legend items are toggled (no page reload)
• When filters match defaults, parameters are omitted for clean URLs
• Composes with existing ?poi= and ?tab= URL features

URL Parameters

Parameter	Example	Purpose
types	?types=trail,historic	Show only listed POI types
boundaries	?boundaries=123,456	Show only listed boundary IDs
layers	?layers=trails,rivers	Toggle trail/river/water-taxi overlays

Closes #531

Test plan

• /?types=trail,historic shows only those POI types
• Toggling legend items updates URL in address bar
• Copy URL → new tab → same filter state
• /?poi=stanford-hostel&types=trail composes correctly
• Clean / works as normal (no params, default state)
• Verified in browser on localhost:8081

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request implements shareable legend filters by parsing 'types', 'boundaries', and 'layers' query parameters from the URL on load to override default filter states, and syncing subsequent user toggles back to the URL. A critical bug was identified in App.jsx where map layers are unintentionally hidden when loading a URL that only filters POI types due to incorrect initialization of 'urlLayersRef.current'.

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[gemini-code-assist]

There is a bug here where urlLayersRef.current is initialized to an empty Set if typesParam is present but layersParam is absent (due to the if (typesParam || layersParam) check). This causes all map layers (trails, rivers, water-taxis) to be hidden by default when loading a URL that only filters POI types (e.g., /?types=trail).\n\nInstead, we should only initialize urlLayersRef.current if layersParam is explicitly provided in the URL query parameters.

    const layersParam = params.get('layers');\n    if (layersParam !== null) {\n      urlLayersRef.current = new Set(layersParam.split(',').map(l => l.trim()).filter(Boolean));\n    }