Fix resume when stream duration is initially unknown

[windoze95]

Summary

• preserve saved resume positions while a progressive stream reports an unknown duration
• retain the known native-duration cap so stale server metadata cannot misplace playback
• share the corrected target calculation across online and offline playback
• add regression coverage for unknown, known, and near-zero resume targets

Root cause

Progressive iOS streams can report a duration of zero immediately after initialization. The player screen used that transient zero as the resume upper bound, converting a valid saved position into a seek to the beginning before the controller’s native zero-duration seek fallback ran. Resume only worked when duration metadata happened to arrive early.

Validation

• flutter analyze --fatal-infos --fatal-warnings
• flutter test (230 tests)
• flutter build web --release