fix(accounting): 'Save & mark paid' actually pays; incoming invoices appear in tax/export

#1 Triage 'Save & mark paid' now marks the invoice paid directly (categorize +
   markInboundPaid with the entered reference) instead of opening the pay dialog
   and leaving it unpaid. Removed the PayModal chain.
#2 Cost side missed captured incoming invoices: the query required
   currency='CHF', but email/upload invoices often have a null currency →
   silently excluded. Now include null-currency rows (treated as the report
   currency). Also replaced COALESCE(invoice_date, created_at) with a split
   date filter (invoice_date BETWEEN, else created_at range) to avoid the
   mixed date/timestamp comparison risk on Postgres. Same fix in the ledger
   export (buildPostings).
en/de: categorizedPaidToast.

frontend/src/i18n/locales/de.json

@@ -3567,7 +3567,8 @@
       "confirmPaid": "Als bezahlt markieren",
       "paid": "Bezahlt",
       "paidToast": "Als bezahlt markiert.",
-      "categorizedToast": "Kategorisiert."
+      "categorizedToast": "Kategorisiert.",
+      "categorizedPaidToast": "Kategorisiert und als bezahlt markiert."
     },
     "expense": {
       "kind": "Art",

frontend/src/i18n/locales/en.json

@@ -3567,7 +3567,8 @@
       "confirmPaid": "Mark paid",
       "paid": "Paid",
       "paidToast": "Marked as paid.",
-      "categorizedToast": "Categorized."
+      "categorizedToast": "Categorized.",
+      "categorizedPaidToast": "Categorized and marked paid."
     },
     "expense": {
       "kind": "Type",