feat: user blocking (block/unblock + blocked list)

[bmc08gt]

What's included

• Block from a tip DM — tapping the nav title or the in-transcript profile card opens a new profile screen (avatar, display name, "Joined ", and a Block row). Block shows a destructive confirmation, calls BlockUser, and returns to Tips with the conversation hidden. For tip DMs this takes precedence over the contact-card action.
• Blocked screen — a beta-gated "Blocked" row in My Account lists blocked users with blurred avatars; tapping a row confirms and calls UnblockUser.
• Feed hiding — the blocklist is the source of truth for who is blocked and reconciles each conversation's isHidden flag; the feed (and Spotlight index) filter on isHidden only. Reconciliation runs on block/unblock, on blocklist refresh, and after every feed load, so a stale server reload can't un-hide a blocked chat. Unblocking restores the conversation with its full history.
• Persistence & sync — a BlocklistTable cache (SQLiteVersion 27→29) backs the Blocked screen offline and refreshes on foreground.
• Client wiring — the already-generated Flipcash_Blocklist_V1_Blocklist service is wired into FlipClient; ErrorBlocklist conforms to TransportClassifiableError (pinned by a test). Block/unblock failures surface a user-facing error alert (copy matched to Android) in addition to Bugsnag capture.
• Proto — re-vendored the Core protos to pick up profile.v1 join_ts, surfaced as Profile.joinedAt.