feat: add CSV export for visitor log

[AchieverSana]

What does this change?

Adds a CSV export for the visitor log (closes #3).

• New GET /api/visitors/export?from=&to= endpoint in backend/routes/visitors.js, gated to manager and readonly roles server-side
• New "Export CSV" button on the Visitor Log page, next to the date filter, visible only to manager/readonly
• CSV includes: name, resident visited, purpose, sign-in time, sign-out time, signed in by
• Correctly escapes quotes/commas so it opens cleanly in Excel/Sheets

How did you test it?

Ran the app locally end-to-end: signed in a test visitor, exported the CSV for that date, and confirmed it opens cleanly in Excel with the correct row data. Verified the button is hidden for non-manager/readonly roles.

Checklist

• npm run lint and npm test pass in frontend/
• No clinical records are hard-deleted, and MAR entries stay append-only (see the rules that can't be broken)
• New API endpoints check the user's role server-side
• Tap targets are still at least 44px if you touched the UI
• No real keys, secrets, or resident data in the diff

Note on the audit-trail acceptance criterion

The issue mentions the export action should be recorded in an activity log. This repo doesn't currently have a dedicated audit-trail table for actions like this (only a care-notes activity_logs table for a different purpose). For now the export is logged server-side via console.log (who exported, what range, when). Happy to add a proper persisted audit table if you'd like a specific approach — didn't want to guess at schema design unprompted.

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