Refactor timeline calculation to avoid state mutation

[egeozcan]

This change addresses a refactoring request in `src/components/timeline.js`. Previously, `calculateColumns` would mutate `this.columns` directly, making it difficult to compare the new value with the old value before taking actions (like fetching new data). The calculation logic has been moved to a pure function pattern, and call sites now handle state updates and side effects only when necessary.

Key changes:

1. Modified `calculateColumns`: It now strictly returns the calculated column count based on element width.
2. Updated `init`: Explicitly assigns the result of `calculateColumns` to `this.columns`.
3. Refined `ResizeObserver`: Uses `calculateColumns` to get the potential new column count, compares it with the current state, and only updates/re-fetches if there is a change.
4. Added Unit Tests: Created `src/components/timeline.test.js` using Vitest/Bun to ensure the logic remains correct and respects boundaries.

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PR created automatically by Jules for task 1714110012230771213 started by @egeozcan

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