MCP Apps: accounting read views (reconciliation, unpaid invoices, prepaid, recurring)

[jernejk]

Part of MCP Apps (#78). Read-only, accounting feature pack only.

Tables over the accounting tools: invoice list with receipts and outstanding amounts (list_invoices, get_invoice_receipts, get_client_outstanding), unpaid invoices by client, prepaid status, recurring invoice list with unit and next date, client rates. Sortable columns, per-client filter, export to CSV from the page (client-side, from the tool result).

Only registered when the accounting feature is enabled, same gate as the tools. No writes.