Opal: web OAuth import, thread splitting, TID rkeys

Web OAuth import flow. opal-web now has a full OAuth import flow: sign in with AT Protocol, authorise, and import posts directly from the browser. Includes a publisher, rate limiter, and import UI. Landing page got about page and import footer link.

Post threading. Long posts now thread as replies instead of truncating. The publisher splits content at natural boundaries and creates a reply chain.

TID-based record keys. Switched from random rkeys to TID-based ones via @ewanc26/tid, giving deterministic, sortable record keys aligned with AT Protocol conventions.

Parser rewrite. Rewrote parsers based on export format research — more accurate parsing of Twitter, Mastodon, and Threads export data.

Other fixes. Removed fs/promises from the library entry point (was breaking browser builds). Added request timeouts to atproto package to prevent serverless function hangs. Fixed opal-web build order (tid must build before opal). Added missing dark theme tokens to opal-web layout. Added Opal logo.