fix(offers): align customer filter with the search field

The customer Select was wrapped in <Field label> while the search was bare, so in the flex-end FilterBar the Field's label + mb:2 pushed the select ~16px above the search input. Drop the Field wrapper (matching the bare list-page filter pattern used on AuditLog/Warehouse/Orders) and make the default option self-describing ("Všichni zákazníci"). Both controls now sit on the same line.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/pages/Offers.tsx

@@ -740,22 +740,20 @@ export default function Offers() {
           />
         </Box>
         <Box sx={{ flex: "0 1 240px" }}>
-          <Field label="Zákazník">
-            <Select
-              value={customerFilter === "" ? "" : String(customerFilter)}
-              onChange={(value) => {
-                setCustomerFilter(value ? Number(value) : "");
-                setPage(1);
-              }}
-              options={[
-                { value: "", label: "Všichni" },
-                ...(customers ?? []).map((c) => ({
-                  value: String(c.id),
-                  label: c.name,
-                })),
-              ]}
-            />
-          </Field>
+          <Select
+            value={customerFilter === "" ? "" : String(customerFilter)}
+            onChange={(value) => {
+              setCustomerFilter(value ? Number(value) : "");
+              setPage(1);
+            }}
+            options={[
+              { value: "", label: "Všichni zákazníci" },
+              ...(customers ?? []).map((c) => ({
+                value: String(c.id),
+                label: c.name,
+              })),
+            ]}
+          />
         </Box>
       </FilterBar>