fix(email): pick the only recipient when none is starred

Creating a recipient never makes it the default, so a single-Kindle setup
opened the send sheet with nobody ticked and Send disabled. Tick the lone
recipient in that case; a starred default still wins.

Ref #1443

client/src/features/email/components/SendBookDialog.vue

@@ -106,8 +106,7 @@ watch(
     loaded.value = false
     await Promise.all([fetchRecipients(), fetchGroups(), fetchProviders(), fetchTemplates(), fetchPreferences()].map((p) => p.catch(() => undefined)))
     loaded.value = true
-    const defaultRecipient = preferences.value?.defaultRecipientId
-    selectedIds.value = defaultRecipient && recipients.value.some((r) => r.id === defaultRecipient) ? [defaultRecipient] : []
+    selectedIds.value = initialRecipientIds()
     providerId.value = preferences.value?.defaultProviderId ?? providers.value[0]?.id ?? null
   },
   { immediate: true },
@@ -115,6 +114,14 @@ watch(
 
 onBeforeUnmount(stopPolling)
 
+/** The starred recipient, or the only one when nobody starred it: creating a recipient never makes it the default. */
+function initialRecipientIds(): number[] {
+  const defaultId = preferences.value?.defaultRecipientId
+  if (defaultId && recipients.value.some((r) => r.id === defaultId)) return [defaultId]
+  const [only, ...rest] = recipients.value
+  return only && rest.length === 0 ? [only.id] : []
+}
+
 /** What each picked recipient gets, and the line a row shows when that differs from the rest. */
 function exceptionFor(recipient: EmailRecipient): { text: string; warn: boolean } | null {
   if (singleBookId.value === null || files.value.length === 0) return null

client/src/features/email/components/__tests__/SendBookDialog.spec.ts

@@ -120,6 +120,31 @@ describe('SendBookDialog', () => {
     expect(mockState.sendBook).toHaveBeenCalledWith({ bookIds: [12], recipientIds: [10], providerId: 3, templateId: undefined, fileId: undefined })
   })
 
+  it('picks the only recipient when nobody starred one', async () => {
+    recipients.value = recipients.value.slice(0, 1)
+    groups.value = []
+    preferences.value = null
+    mountSheet()
+    await flushPromises()
+
+    expect(recipientRow('Kindle Paperwhite').getAttribute('aria-checked')).toBe('true')
+
+    buttonByText('Send').click()
+    await flushPromises()
+
+    expect(mockState.sendBook).toHaveBeenCalledWith(expect.objectContaining({ recipientIds: [10], providerId: 3 }))
+  })
+
+  it('leaves the choice open when several recipients exist and none is starred', async () => {
+    preferences.value = null
+    mountSheet()
+    await flushPromises()
+
+    expect(recipientRow('Kindle Paperwhite').getAttribute('aria-checked')).toBe('false')
+    expect(recipientRow('Ana Beck').getAttribute('aria-checked')).toBe('false')
+    expect(buttonByText('Send').disabled).toBe(true)
+  })
+
   it('says what a recipient gets only when it differs from the rest', async () => {
     mountSheet()
     await flushPromises()