feat(notifications): show thread root context

[hmans]

Summary

• expose a bounded, current thread-root message excerpt on Notifications 2.0 groups
• hydrate excerpts only after current notification target visibility validation, without copying message text into persisted notification state
• render the excerpt as a compact thread reference so separate threads in one room remain distinguishable
• update ADR-072, FDR-012, generated clients, and ConnectRPC reference docs

This PR is stacked on #1967 and should merge after it.

API compatibility

This adds the optional NotificationGroup.thread_root_message_excerpt field. The change is additive against the parent branch; older clients ignore it, while newer clients gracefully omit the reference when talking to a server that does not return it.

Verification

• mise codegen-proto
• buf breaking proto --against '.git#branch=origin/hmans/notifications-2-0-details,subdir=proto'
• mise test-cli
• mise test-frontend (125 server files / 1,137 tests; 159 client files / 1,597 tests; 60 Storybook files / 180 tests; performance comparison passed)
• mise license-check
• pnpm --dir apps/frontend check (0 errors, 0 warnings)
• focused ESLint, Go hydration/excerpt tests, and notification-page browser tests
• Chrome DevTools verification with two real thread mentions in one room

Decision records

• ADR-071
• ADR-072
• FDR-012