feat(invoices): surface monthly/manual accumulator drafts in the Bills list

Manual/monthly-cadence customers accumulate logged hours into one running
draft invoice (is_monthly_draft, migration 128). That draft gets a real
invoice number and stamps the hours ("Billed: R-2026-0026"), but listInvoices
hid is_monthly_draft rows from the main list — so the invoice looked lost even
though it existed on the customer's monthly-queue card. It also carried status
'scheduled' despite never auto-sending on manual cadence, reading misleadingly
as "Scheduled".

- Bills list now opts into drafts via a new `includeDrafts` query param
  (GET /admin/invoices → listInvoices includeMonthlyDrafts). Pickers/sub-lists
  that reuse billsService.list leave it off, so they're unaffected.
- Draft rows render a distinct "Draft" badge instead of "Scheduled"
  (transformInvoice already exposes isMonthlyDraft).
- The hours "Billed: R-…" chip now links straight to its invoice.
- i18n: bills.status.draft (de "Entwurf", en "Draft").

backend/src/routes/adminInvoices.js

@@ -342,6 +342,7 @@ router.get(
     query('customerAccountId').optional({ values: 'falsy' }).isInt({ min: 1 }),
     query('sourceQuoteId').optional({ values: 'falsy' }).isInt({ min: 1 }),
     query('unpaidOnly').optional({ values: 'falsy' }).isBoolean(),
+    query('includeDrafts').optional({ values: 'falsy' }).isBoolean(),
     query('q').optional({ values: 'falsy' }).isString().isLength({ max: 255 }),
     query('sort').optional({ values: 'falsy' }).isIn(['newest', 'oldest', 'issue_asc', 'issue_desc', 'due_asc', 'due_desc', 'value_asc', 'value_desc', 'customer_asc', 'customer_desc']),
     query('page').optional({ values: 'falsy' }).isInt({ min: 1 }),
@@ -358,6 +359,9 @@ router.get(
         customerAccountId: req.query.customerAccountId ? parseInt(req.query.customerAccountId, 10) : null,
         sourceQuoteId: req.query.sourceQuoteId ? parseInt(req.query.sourceQuoteId, 10) : null,
         unpaidOnly: req.query.unpaidOnly === 'true' || req.query.unpaidOnly === true,
+        // Surface running monthly/manual accumulator drafts (hidden by
+        // default per migration 128) when the Bills list explicitly asks.
+        includeMonthlyDrafts: req.query.includeDrafts === 'true' || req.query.includeDrafts === true,
         q: req.query.q,
       },
       sort: req.query.sort || 'issue_desc',