Respect HTTP caching headers for tile requests

[dougborg]

Context

The OSM tile usage policy requires:

1. Cache tiles locally per HTTP caching headers (Cache-Control, Expires, ETag) with a minimum 7-day fallback when headers aren't present
2. Use conditional requests (If-None-Match, If-Modified-Since) for expired tiles to avoid re-downloading unchanged tiles
3. Never send Cache-Control: no-cache or similar bypass directives (we're compliant here — we don't send these)

Current behavior

• tiles_from_remote.dart fetches tiles with a plain GET and only checks the status code — response caching headers are ignored entirely
• No persistent tile cache with TTL tracking exists (flutter_map's in-memory cache evicts on map rebuild)
• Expired tiles are re-fetched unconditionally (no ETag/304 flow)

Expected behavior

• Parse Cache-Control / Expires from tile responses and store tiles on disk with expiration metadata
• Default to 7-day cache when response headers are absent
• On cache expiry, send conditional requests (If-None-Match / If-Modified-Since) and handle 304 Not Modified

Notes

This applies to all tile servers the app talks to, not just tile.openstreetmap.org.