Fix #10: Add data-layer caching to eliminate repeated Firestore reads

[cak]

Fixes #10 by introducing a small data-access cache to reduce repeated Firestore reads during navigation.

What changed

• Added src/lib/db/cache.ts

  • Namespaced keys (htw:v1)
  • In-memory cache for same-session reuse
  • localStorage persistence with TTL
  • Safe JSON parsing and error handling

• Updated DB access functions (conferences, events, tags, speakers)

  • Cache-first reads
  • Firestore fallback on miss or stale data

• Event lookups

  • Prefer cached full conference event list before individual fetches

• Speaker lookups

  • Reuse cached full speaker lists and individual entries

Behavior

• First load still fetches from Firestore
• Subsequent navigation reuses cached data
• Schedule ↔ event navigation is now effectively instant
• Cache is time-bounded and safely ignored if stale or invalid