refactor(web): split member.tsx into hooks and components

[naasanov]

Prep work for the chat transcript export (#73). Pure refactor — no behavior changes, so the transcript PR stacked on top stays reviewable.

member.tsx had reached 1235 lines holding the chat, the composer, patient controls, referrals and checklists. This splits it along the boundaries main already established with ChecklistWidget / useChecklists / CollapsibleSection: hooks own the Firestore subscriptions, components own their own UI and handlers.

New file	Contents
hooks/useChat.ts	message state, live listener, pagination, sender profiles, read receipts, send
hooks/usePatientProfile.ts	patient doc subscription, current user profile, social worker list
components/ChatPanel.tsx	chat column
components/MessageBubble.tsx	one message + chunk grouping
components/ChatComposer.tsx	textarea, image picker, send
components/PatientControls.tsx	assignment / status / verify-deny
utils/format_datetime.ts	shared message-timestamp + person-name formatters

member.tsx ends as a 156-line layout shell.

Deliberately not done here

The live-listener bug is carried across unchanged. The onSnapshot handler replaces the whole message list with only the newest page, discarding older pages loaded by loadOlderMessages. It's filed as #77 with a comment pointing at it from the new code. Fixing it inside a 1000-line move would have made this diff impossible to review — and #77 needs human confirmation of the repro first.

The ad-hoc date formatters in _index.tsx and calendar.tsx were left alone. They looked like duplicates of formatMessageDateTime but aren't: one renders date-only with a year, another a full weekday with no time, the third date+time with no year. Unifying them would have changed what renders in three places.

Verification

npm run typecheck, npm run lint, npm run build all clean.

Behavior preservation was checked mechanically as well as by hand: every one of the 129 string literals in the original member.tsx still appears somewhere in the new file set.