feat: add smooth fade-in transition for route polylines

[sejalkutriyar]

Summary

• Add opt-in opacity-based fade-in animation for route polylines in the OpenStreetMap provider using requestAnimationFrame.
• Enable the animation only for route-shape rendering flows (keeping other polyline callers unchanged).

Implementation details

• Added animatePolylineOpacity helper to smoothly animate polyline opacity on the client.
  • Includes safety guards (SSR, map/layer presence, and layer still attached).
• Updated createPolyline(points, options) to accept { animate } (default: false).
  • When enabled, the polyline starts at opacity 0 and fades to the target opacity.
• Route flows that display primary route shapes now pass { animate: true }:
  • route modal
  • search route selection

Testing

• npm run lint (passes)
• npm test -- src/tests/lib/serverCache.test.js --run (passes, all 37 tests)
• Manual verification done on local dev map view.

[CLAassistant]

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