Render real homepage stats

[VidurShah]

Problem

The home screen's stat grid rendered hardcoded placeholders — newMessageCount = 12, completedChecklists = 10 / 12, activeReferrals = 11. Only the Calendar tile was wired to real data, so a patient saw three numbers that had nothing to do with their account. Closes #72.

The Checklists and Referrals tiles were straightforward: ChecklistService.streamCurrentUserChecklists and ReferralService.streamCurrentUserReferrals already existed and already scoped to the current user. Unread messages was the one that needed new plumbing, and it came with two constraints:

• Filtering on both isRead and senderId server-side would need a new composite index. firestore.indexes.json only carries timestamp field overrides for the messages subcollection.
• Counting alone gives a number that never goes down. Nothing in mobile/lib has ever written isRead — the web console is the only side implementing read receipts today, so an unread badge would have stuck at its first value forever.

Changes

• lib/services/chat_service.dart — added streamUnreadCount(chatId), which filters server-side on isRead only. That equality-only query is covered by the automatic single-field index; the senderId check runs client-side to avoid needing a composite one. streamCurrentUserUnreadCount() wraps it for callers that don't have a chat id — the chat document id is the uid, so no lookup is needed and a chat that doesn't exist yet simply streams 0. Also added markMessagesRead(docs), which batches isRead: true onto messages the current user didn't send.
• lib/pages/home_page.dart — dropped the hardcoded fields and added _buildChatCard, _buildChecklistCard and _buildReferralsCard, each following the shape _buildCalendarCard already used (waiting → Loading..., error or null → a neutral fallback, else the value). Chat reads No new messages / N new message(s); Checklists counts unchecked items across all non-archived lists and reads All caught up / N item(s) left; Referrals counts every non-deleted referral regardless of status and reads No referrals / N referral(s). The services are held as final fields alongside the existing _eventService so the streams are created once per HomePage rather than on every rebuild.
• lib/pages/chat_page.dart — one unawaited markMessagesRead call inside _MessagesListState.build, in the existing docs.length != _lastDocCount block that already handles the auto-scroll side effect. It reuses the docs the list has already streamed, so read receipts cost no extra reads, and it can't loop: flipping isRead changes document contents but not the count, so the guard blocks a re-fire.

firestore.rules already permits this write — each side may flip isRead to true on messages they did not send — so nothing under backend/ changed, and neither did firestore.indexes.json or pubspec.yaml.

Notes / follow-ups

• Read receipts are deliberately best-effort. A failed markMessagesRead is debug-logged and swallowed rather than surfaced — the patient did nothing wrong, and the write retries on the next snapshot anyway.
• The unread definition assumes the two sides never collide on isRead. The web console only marks messages read where senderId !== user.uid, i.e. patient-to-staff; mobile now marks the reverse. The sets are disjoint, so the single shared flag works — but it is a single flag, and a third participant on a chat would break that assumption.
• Worth exercising during review: a message sent from mobile must never count toward the patient's own unread, and a referral whose status changes to completed must not change the count while soft-deleting one does.