app: a download frequency of "Once" no longer blocks submission

useForm treated any falsy value as a missing required field.  "Once" is
frequency 0, so the Channel/Playlist, RSS, and Zim download forms never
became ready and the Download button stayed disabled.  Only null,
undefined, '', false, and NaN count as missing now.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/src/components/Download.cy.js

@@ -127,6 +127,15 @@ describe('<ChannelDownloadForm/>', () => {
         shouldBeInvalid('#url_input');
     });
 
+    it('can be submitted with a frequency of Once', () => {
+        cy.get('#url_input').type('https://example.com/playlist').wait(500);
+        cy.get('#frequency_select').click();
+        cy.contains('[role="option"]:visible', 'Once').should('be.visible').click({force: true});
+        cy.get('#frequency_select').should('have.value', 'Once');
+
+        cy.get('#download_form_download_button').should('not.be.disabled');
+    });
+
     it('can choose different options', () => {
         /*
          * `#download_frequency_selector` and `.item` named the old dropdown and its menu rows.

app/src/hooks/useForm.js

@@ -67,7 +67,11 @@ export function useForm({
             setReady(false);
             return
         }
-        const missingValues = Object.keys(memoizedRequires).filter(i => !_.get(formData, i));
+        // 0 is a real value (a download frequency of "Once"), only empty values are missing.
+        const missingValues = Object.keys(memoizedRequires).filter(i => {
+            const value = _.get(formData, i);
+            return value === null || value === undefined || value === '' || value === false || Number.isNaN(value);
+        });
         if (missingValues.length > 0) {
             console.debug(`Form invalid because it is missing required value: ${missingValues[0]}`);
             setReady(false);

app/src/hooks/useForm.test.js

@@ -727,6 +727,29 @@ describe('useForm', () => {
             });
         });
 
+        it('zero satisfies a required field', async () => {
+            // A download frequency of 0 means "Once".
+            const {result} = renderHook(() => useForm({
+                defaultFormData: {frequency: null},
+                submitter: jest.fn(),
+            }));
+
+            act(() => {
+                result.current.getCustomProps({name: 'frequency', required: true});
+            });
+            await waitFor(() => {
+                expect(result.current.ready).toBe(false);
+            });
+
+            act(() => {
+                result.current.setValue('frequency', 0);
+            });
+            await waitFor(() => {
+                expect(result.current.formData.frequency).toBe(0);
+                expect(result.current.ready).toBe(true);
+            });
+        });
+
         it('addRequires marks field as required', async () => {
             const {result} = renderHook(() => useForm({
                 defaultFormData: {field: ''},