Preserve playback state during scrubbing

[ResonantSpiral]

Summary

• capture the playback state only when a scrub gesture begins
• preserve the original playing or paused state across repeated slider updates
• leave click seeking and relative seeking unchanged

Fixes #305.

Detailed reproduction and source diagnosis: #325.

Testing

• cargo +1.93.0 check --locked
• cargo +1.93.0 test --locked (the project currently defines no unit tests)
• cargo +1.93.0 build --locked
• git diff --check
• manually verified that scrubbing playing media resumes playback
• manually verified that scrubbing paused media remains paused
• manually verified click seeking and relative seeking
• manually exercised MP4, WebM, FLAC, MP3, and OGG media in normal and condensed layouts

cargo +1.93.0 fmt --check still reports the existing import-order difference in the unmodified build.rs. The modified src/main.rs is not reported by the formatter.

AI Assistance

Codex assisted with tracing the seek-state flow, preparing the narrow patch, and running validation. I reviewed and understand the change and can respond to review feedback. The commit message also discloses this assistance.

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