fix(mobile): build a standard geo: URI for external map apps on Android

[Deeds67]

Fixes #1087.

Problem

Tapping the map in the asset detail panel on Android launched geo://lat,lng?z=16&q=lat%2Clng. It was built with Uri(scheme: 'geo', host: ...), which adds a // authority and percent-encodes the comma in q. Google Maps accepts that, but Baidu Maps reads q as search text and Amap fails to open the location.

Change

• New pure helper mobile/lib/utils/geo_uri.dart → geo:lat,lng?q=lat,lng&z=16 (the RFC 5870 / Android intent form).
• exif_map.dart uses it. iOS and the OpenStreetMap fallback are unchanged.

Testing

• mobile/test/utils/geo_uri_test.dart: checks the exact string, negative coordinates, and parse-back (no authority, literal comma).
• Confirmed the test fails against the old construction (Actual: 'geo://31.2304,121.4737?z=16&q=31.2304%2C121.4737').
• dart analyze --fatal-infos and dart format are clean on the touched files.

Not covered

• Not tested on a device with Amap or Baidu installed. The fix makes the URI match the standard form both apps document; it has not been checked end to end.
• Chinese map apps use GCJ-02 / BD-09 coordinates, while EXIF GPS is WGS-84, so the pin may land a few hundred metres off inside mainland China. That is a separate issue and is not addressed here.