feat(settings): let the user set a default mail account

Persist a defaultAccountId preference (SettingsRepository/AppSettings,
following the existing key/field/setter pattern), add a "Default account"
switch to AccountSettingsScreen, and prefer it in ComposeViewModel's
from-account fallback (fromAccountId -> valid default -> first account).
Deleting the default account clears the preference (SettingsRepository
.clearDefaultAccountId), and a stale/foreign id is validated against the
current account list before use so it can never crash or point at a
missing account.

Closes #163

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

app/src/androidTest/kotlin/org/libremail/ui/compose/ComposeScreenTest.kt

@@ -28,6 +28,7 @@ import org.libremail.R
 import org.libremail.contacts.ContactsRepository
 import org.libremail.data.local.AccountDatabase
 import org.libremail.data.settings.AccountSettingsRepository
+import org.libremail.data.settings.SettingsRepository
 import org.libremail.data.settings.SignatureRepository
 import org.libremail.domain.model.Account
 import org.libremail.domain.model.AuthType
@@ -88,6 +89,7 @@ class ComposeScreenTest {
             contactsRepository = ContactsRepository(context),
             accountSettingsRepository = AccountSettingsRepository(database.accountSettingsDao()),
             signatureRepository = SignatureRepository(database.signatureDao()),
+            settingsRepository = SettingsRepository(context),
         )
         composeTestRule.setContent {
             LibreMailTheme(darkTheme = false, dynamicColor = false) {

app/src/androidTest/kotlin/org/libremail/ui/settings/AccountSettingsScreenTest.kt

@@ -19,6 +19,7 @@ import org.libremail.R
 import org.libremail.data.local.AccountDatabase
 import org.libremail.data.local.toEntity
 import org.libremail.data.settings.AccountSettingsRepository
+import org.libremail.data.settings.SettingsRepository
 import org.libremail.data.settings.SignatureRepository
 import org.libremail.data.sync.SyncScheduler
 import org.libremail.domain.model.Account
@@ -71,6 +72,7 @@ class AccountSettingsScreenTest {
             accountSettingsRepository = repository,
             signatureRepository = SignatureRepository(db.signatureDao()),
             syncScheduler = SyncScheduler(Provider { WorkManager.getInstance(context) }),
+            settingsRepository = SettingsRepository(context),
         )
         composeTestRule.setContent {
             LibreMailTheme(darkTheme = false, dynamicColor = false) {