Add locate-me button to zoom map to GPS position

[rastermanden]

Summary

• Adds a crosshair FAB button bottom-right on the map
• Tapping it requests GPS permission and flies the map to the user's position at zoom 15
• Button turns terracotta while acquiring location; shows a toast on error

Test plan

• Tap the locate button — browser requests location permission
• After granting, map flies to current GPS position
• Deny permission → toast appears with error message
• Works on mobile and desktop

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