Add Gmail-style multi-select build deletion

[VeldtJumper]

Summary

• enter contextual selection mode by long-pressing a build
• show row checkboxes and an X / selected count / Delete top bar
• confirm bulk deletion with duplicate prevention, partial-failure handling, Back behavior, and accessible checked semantics
• send selected builds through a bounded backend batch API instead of one HTTP request per build

Batch deletion API

• POST /v1/apps/{appId}/versions/batch-delete
• accepts up to 100 build IDs per request
• validates ownership and updates Firestore versions, storage accounting, and the parent app aggregate in one transaction
• cleans up B2 objects sequentially to avoid a deletion fan-out
• client chunks selections above 100 into sequential requests; selecting 50 builds uses one request

Validation

• full server test suite
• Android host tests, including selection, navigation, deletion coordination, and API batching tests
• Android debug and release assembly
• Android lintDebug
• git diff checks
• read-only code review found no actionable regressions
• CI build passed, including web-target validation

No connected Android device was available for manual interaction testing.