feat: sort slides listing by date DESC, title DESC as tiebreaker

[claudinethelobster]

Summary

Updates the slides listing to use a deterministic sort order:

1. Date DESC (newest first) — using the timestamp ISO field from each slide's metadata.json
2. Title alphabetical DESC — as a tiebreaker when timestamps are equal

Changes

app/slides/slidesData.ts

• Added exported compareSlides(a, b) comparator function
• Updated getAllSlides() to .sort(compareSlides) before returning

Comparator logic:

• Parses timestamps via Date.parse()
• Valid timestamps sort newer-first (bTime - aTime)
• Tiebreaker: b.title.localeCompare(a.title) (DESC)
• Missing/invalid timestamps fall to the end, with title DESC tiebreaker among themselves

app/slides/slidesData.test.ts (new)

• 7 vitest unit tests covering: newer-first, title-DESC tiebreak, identical slides, missing timestamp fallback, invalid timestamp fallback, two invalid timestamps, and a full array integration test
• All 7 tests pass ✅

package.json

• Added vitest devDependency
• Added "test": "vitest run" script

Test Results

✓ app/slides/slidesData.test.ts (7 tests) 10ms
Test Files  1 passed (1)
Tests  7 passed (7)

[samholmes]

Remove vitest. If you want to write tests, write it in a dedicated tests/ directory and not app/ directory and use bun's test framework (less dependencies).