feat(fleet): validate vehicle plate numbers against a letters-and-digits format

Plate, power-plate and trailer accepted any free text — a vehicle could be
saved with a plate of "assadasd". They must be letters, a hyphen, then digits,
like ET-9875 or AA-8642.

The server now enforces it on CreateVehicleDto (and UpdateVehicleDto via
PartialType): each plate is trimmed and upper-cased, then matched against
^[A-Z]{2,3}-\d{2,6}$, so "et-9875" is accepted and stored as ET-9875 while an
empty optional trailer/power plate still passes.

The fleet form gains the same check inline: FleetFormFieldDef takes an optional
pattern, the dialog tests it on submit against the upper-cased value, and the
vehicle config points plate and trailer at a regex that mirrors the server's.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/edr-freight-web/backoffice/src/components/fleet/FleetFormDialog.tsx

@@ -249,6 +249,16 @@ const FleetFormDialog = ({
           }
         }
       }
+
+      // Format check (e.g. plate numbers). Skipped for an empty optional field —
+      // "required" above already owns the empty case. Upper-cased to match the
+      // server, which stores plates upper-case.
+      if (field.pattern && stringValue && stringValue !== FLEET_SELECT_NONE) {
+        const candidate = field.pattern.uppercase === false ? stringValue : stringValue.toUpperCase();
+        if (!field.pattern.regex.test(candidate)) {
+          next[field.name] = field.pattern.message;
+        }
+      }
     });
     setErrors(next);
     return Object.keys(next).length === 0;