fix: clamp timeline drag selection to 1s minimum

[sshane]

Summary

Sub-second drag selections produced inconsistent state — zoom/loop held millisecond-precision bounds while the URL collapsed (`Math.floor(start/1000) === Math.floor(end/1000)`) and dropped the range. Reload would lose the section even though Redux still had it.

Round the drag's start/end to whole seconds (matching URL precision) and ensure end > start, so any drag becomes at least a 1-second section. No separate ms-precision internal state to keep in sync with the URL.

Test plan

• Tiny drag (under a second) → resulting section is exactly 1s wide and reflected in the URL
• Normal drag (e.g. 5s span) → unchanged, still rounds to whole seconds
• Drag at second 0 → still creates a section (unrelated, but worth eyeballing alongside fix: timeline selections starting at offset 0 #592)
• `pnpm lint` clean
• `pnpm test` passes (30/30)

🤖 Generated with Claude Code

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