Local History Cache & Storage

This PR introduces a comprehensive local caching system for message history across all Orbit clients. It covers the full specification for on-device persistent caching, including progressive rendering, platform-specific implementations (IndexedDB on web, SQLite on desktop), and a user-facing storage management interface.

What this means:

• On-device persistent cache (IndexedDB on web, SQLite on desktop) that caches all messages the client has seen
• Cache-first prefill - buffers open instantly from disk, with network reconciliation filling only the delta
• Progressive rendering - large buffers mount incrementally to keep the main thread responsive
• Sliding window pagination - users can scroll arbitrarily through cached history while keeping the DOM bounded
• Storage management - Settings -> Storage UI showing per-buffer stats, eviction controls, JSON export, and configurable per-buffer caps

Why This Matters:

• Massively reduces server load by serving scrollback from disk instead of repeated chathistory requests
• Messages persist beyond the server's retention window, giving each client its own incremental archive
• Foundation for client-side search indexing (upcoming follow-up)
• Enables reconnection deltas instead of full re-pulls
• Maintains IRC's trust model - caches only what the server delivered, keyed by server-asserted msgid

Platform Differences

See the full spec in spec/04-clients/04-local-cache.md for implementation details, configuration constants, and honest environment constraints.