feat: add LRU eviction and size cap to image cache

[Sughter99]

Closes #677

What this PR does

Adds LRU eviction and a hard 100 MB size cap to the image cache,
preventing unbounded disk usage during heavy course catalog browsing.

Changes

src/utils/imageCache.ts

• Added EVICTION_THRESHOLD_BYTES = 80 MB (80% of MAX_CACHE_SIZE_BYTES)
• Updated evictLRU() to accept targetBytes parameter (defaults to 70% of max)
• Added maybeEvict() — called after every prefetch write, triggers LRU
  eviction when cache reaches 80 MB threshold
• Added clearNonCritical() — removes all entries without clearing the
  metadata structure, called on critical memory pressure
• Added getCacheSizeBytes() — exposes current cache size as a metric
• Added getEvictionThresholdBytes() — exposes threshold for testing

src/services/memoryPressureService.ts

• Updated handleHighMemoryPressure() to call clearNonCritical() instead
  of clearCache() to preserve metadata structure during pressure events

src/tests/utils/imageCacheLRU.test.ts (new)

• 6 tests covering:
  • Eviction threshold is 80% of 100 MB
  • getCacheSizeBytes returns 0 on empty cache
  • Eviction fires when 81st MB is pushed past 80 MB threshold
  • clearNonCritical removes all entries and resets size to 0
  • getCacheSizeBytes returns correct total after prefetch
  • Cache never exceeds 100 MB after many prefetches

Acceptance Criteria

• Image cache does not exceed 100 MB on disk
• LRU eviction fires when cache reaches 80 MB
• Critical memory pressure clears all non-pinned cached images
• Unit test confirms eviction at 80% threshold
• 6 tests passing

[drips-wave]

@Sughter99 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[RUKAYAT-CODER]

Thank you for contributing to the project.