fix(timeline): resolve duplicate trailing timestamps and key collisions on frames strip

[trivikramkalagi91-commits]

Description
This PR resolves a systemic rounding flaw in the ThumbnailStrip component where trailing video frames output duplicate formatted timestamps (e.g., displaying 0:10 twice on an 11-second video or creating structural layout duplication).

The mathematical bounding constraints previously introduced collision states when video durations did not divide cleanly into the set interval steps. This fix introduces a string-based tracking layout using a native Set against the formatted output of formatTime(), ensuring every generated frame is strictly unique in both value and presentation.

Related Issue
Fixes #1069

Type of Contribution

• Bug fix
• New feature
• Documentation update
• Refactor
• GSSoC contribution

Participant Info

• GitHub username: trivikramkalagi91-commits
• Contribution level (Beginner/Intermediate/Advanced): Intermediate

Screenshots:
BEFORE:
one extra frame

AFTER:
only 3 frames

Checklist

• I have read the contribution guidelines
• My changes follow the project structure
• I have tested my changes in Chrome, Firefox, and Safari
• bun run lint passes (no ESLint errors)
• bunx tsc --noEmit passes (no TypeScript errors)
• New interactive elements have aria-label / accessible names
• No console.log statements left in
• This PR is related to a valid issue

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[trivikramkalagi91-commits]

Hi @magic-peach I've submitted this PR to resolve issue #1069. It fixes the systemic rounding flaw causing duplicate trailing timestamps on shorter video timelines. Local tests, linting, and type-checking all pass successfully. Ready for your review (and Vercel authorization).