Group inbox replies into conversations

[olegberman]

What changed

• resolve replies through In-Reply-To and References instead of subject-only grouping
• list one latest-activity row per conversation with aggregate unread, star, attachment, and message-count state
• apply read, star, archive, and trash actions across the accessible conversation
• rebuild existing subject-grouped history with a forward-only D1 migration

Why

Inbound replies were stored in separate or overly broad subject-based groups, so the inbox surfaced
individual messages instead of stable conversations. This uses email reply headers as the durable
thread identity and keeps same-subject unrelated messages separate.

Validation

• CI=true pnpm check
• CI=true pnpm deploy:dry-run
• rendered verification at 1440x900 and 390x844
• verified a three-message inbound/outbound reply chain renders as one inbox row and opens
  chronologically