feat(ui): show per-source data freshness

[Sam-24-dev]

Summary

• replace the misleading global freshness label with GitHub, Stack Overflow, and Reddit source-level UTC dates
• derive freshness only from mapped dataset_summaries.updated_at_utc values; never from generated_at_utc
• expose the same source dates in accessible health metadata

Mapping

• github_ → GitHub
• so_ → Stack Overflow
• reddit_ → Reddit
• derived and cross-source datasets are intentionally excluded

Validation

• flutter test (145 passing)
• focused model, accessibility, and mobile dashboard smoke tests
• flutter analyze
• flutter build web --release
• staged detect-secrets scan

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