refactor: maps

[jamesarich]

Draft

This commit restructures the map-related code to support different map providers (OSMDroid for F-Droid, Google Maps for Google Play).

Key changes:

• Moved OSMDroid-specific map files to the fdroid source set.
• Created placeholder files for Google Maps in the google source set.
• Added Google Maps Compose dependencies and the secrets plugin to build.gradle.kts.
• Added local.defaults.properties for API key management.
• Introduced MAP_STYLE_ID constant.
• Added "Unknown" string resource.
• Created LocationUtils.kt in the google source set with GPS formatting and calculation utilities.