fix(dkv): fix Kennzeichen matching and NaN/invalid km values in Excel export

Three issues fixed:

1. Kennzeichen normalization: DKV PDF extracts plates without hyphens
   ("GP JL 740E" vs CSV-imported "GP-JL 740E"). Added _normalizeKennzeichen()
   which strips hyphens, spaces, and dots before lookup — resolves vehicle
   master match failure that caused Marke/Modell/Fahrer to appear empty.

2. Empty-string NaN: parser used ?? '0' which doesn't catch empty strings,
   causing parseDE('') = NaN. Changed to || '0' for km, menge, and totals.

3. Invalid km values: EV charging rows from DKV have misaligned columns —
   km position contains a decimal price (e.g. 18.64 EUR or kWh). Added
   sanity check: non-integer km values are written as null (empty cell)
   instead of a misleading decimal. ExportRow.kilometerstand is now number|null.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/dkv/dkv-parser.service.ts

@@ -207,13 +207,14 @@ export class DkvParserService {
     return dates.map((date, i) => ({
       lieferdatum: date,
       ort: orts[i] ?? '',
-      // German number format: "19.234,56" → 19234.56 (strip dots, replace comma)
-      kilometerstand: parseDE(kms[i] ?? '0'),
+      // German number format: "19.234,56" → 19234.56 (strip dots, replace comma).
+      // Use || '0' (not ??) so empty strings also fall back to '0' (avoids NaN).
+      kilometerstand: parseDE(kms[i] || '0'),
       produkt: products[i] ?? '',
-      menge: parseDE(quantities[i] ?? '0'),
+      menge: parseDE(quantities[i] || '0'),
       einheit: units[i] ?? '',
-      netto: parseDE(totals_netto[i] ?? '0'),
-      brutto: parseDE(totals_brutto[i] ?? '0'),
+      netto: parseDE(totals_netto[i] || '0'),
+      brutto: parseDE(totals_brutto[i] || '0'),
     }));
   }
 }