Export dialog improvements (#24787)

* show validation errors in multi-camera export

Clearing part of a time field in the export time picker set the range to NaN, while the picker kept showing a valid fallback time. The multi-camera Export button then stayed disabled with no explanation, and non-admins could never start an export because a case name was required even though they can't see the case field. The picker now ignores incomplete time values. The multi-camera tab shows inline errors for an invalid range, no selected cameras, and a blank case name, only requires a case name from admins, and prefills a default case name.

* add viewer export test and remove stale non-admin case text

* fix test

web/src/components/overlay/CustomTimeSelector.tsx

@@ -151,6 +151,11 @@ export function CustomTimeSelector({
               step={isIOS ? "60" : "1"}
               onChange={(e) => {
                 const clock = e.target.value;
+
+                if (!clock) {
+                  return;
+                }
+
                 const [hour, minute, second] = isIOS
                   ? [...clock.split(":"), "00"]
                   : clock.split(":");
@@ -217,6 +222,11 @@ export function CustomTimeSelector({
               step={isIOS ? "60" : "1"}
               onChange={(e) => {
                 const clock = e.target.value;
+
+                if (!clock) {
+                  return;
+                }
+
                 const [hour, minute, second] = isIOS
                   ? [...clock.split(":"), "00"]
                   : clock.split(":");