fix(forum): preserve resolved authors across restarts

[wesbillman]

Summary

• merge the loaded current profile into forum author lookups so your own posts keep a resolved label after reload
• recover the attributed author from the first p tag for relay-signed forum posts and replies
• document SPROUT_RELAY_PRIVATE_KEY in .env.example for stable relay signing in local dev

Context

DMs and channels were already stable because they use different rendering and signing paths. Forum was the weak spot: it relied on a separate batch profile lookup for self-resolution, and older relay-signed forum events can still carry the real author in the first p tag even when the serialized pubkey path has drifted.

Testing

• source ./bin/activate-hermit && cd desktop && pnpm typecheck
• source ./bin/activate-hermit && cd desktop && pnpm check
• git push pre-push checks passed (desktop-check, desktop-build, desktop-tauri-check, rust-fmt, rust-clippy, rust-tests)