fix: resolve default sender to canonical identity on local-part login

When authenticating with a local-part username (e.g. 'user' instead of
'user@domain.tld') on Stalwart 0.15.x, the default sender could resolve
to an alias identity instead of the canonical mailbox address.

- Add emailMatchesUsername() helper that matches local-part usernames
  against full email addresses (e.g. 'user' matches 'user@domain.tld')
- Prefer canonical identities (mayDelete=false) over aliases as tiebreaker
- Add preferredPrimaryId to identity store (persisted to localStorage)
  so users can explicitly set their default sender
- Add 'Set as Primary' star button in identity manager modal
- Fix sendEmail() fallback identity resolution for local-part usernames
- Add i18n strings for all 8 supported locales

Fixes #43

locales/de/common.json

@@ -519,6 +519,7 @@
     "identity_created": "Identität erfolgreich erstellt",
     "identity_updated": "Identität erfolgreich aktualisiert",
     "identity_deleted": "Identität gelöscht",
+    "identity_set_primary": "Primäre Identität aktualisiert",
     "identity_create_failed": "Identität erstellen fehlgeschlagen: {error}",
     "identity_update_failed": "Identität aktualisieren fehlgeschlagen: {error}",
     "identity_delete_failed": "Identität löschen fehlgeschlagen: {error}",
@@ -1367,6 +1368,7 @@
     "delete_confirm": "Diese Identität löschen? Dies kann nicht rückgängig gemacht werden.",
     "cannot_delete": "Diese Identität kann nicht gelöscht werden",
     "primary_identity": "Primär",
+    "set_as_primary": "Als primär festlegen",
     "no_identities": "Keine Identitäten gefunden",
     "display": {
       "reply_to": "Antwort an:",

locales/en/common.json

@@ -519,6 +519,7 @@
     "identity_created": "Identity created successfully",
     "identity_updated": "Identity updated successfully",
     "identity_deleted": "Identity deleted",
+    "identity_set_primary": "Primary identity updated",
     "identity_create_failed": "Failed to create identity: {error}",
     "identity_update_failed": "Failed to update identity: {error}",
     "identity_delete_failed": "Failed to delete identity: {error}",
@@ -1367,6 +1368,7 @@
     "delete_confirm": "Delete this identity? This cannot be undone.",
     "cannot_delete": "This identity cannot be deleted",
     "primary_identity": "Primary",
+    "set_as_primary": "Set as primary",
     "no_identities": "No identities found",
     "display": {
       "reply_to": "Reply-To:",

locales/es/common.json

@@ -519,6 +519,7 @@
     "identity_created": "Identidad creada exitosamente",
     "identity_updated": "Identidad actualizada exitosamente",
     "identity_deleted": "Identidad eliminada",
+    "identity_set_primary": "Identidad principal actualizada",
     "identity_create_failed": "Error al crear identidad: {error}",
     "identity_update_failed": "Error al actualizar identidad: {error}",
     "identity_delete_failed": "Error al eliminar identidad: {error}",
@@ -1367,6 +1368,7 @@
     "delete_confirm": "¿Eliminar esta identidad? Esto no se puede deshacer.",
     "cannot_delete": "Esta identidad no se puede eliminar",
     "primary_identity": "Principal",
+    "set_as_primary": "Establecer como principal",
     "no_identities": "No se encontraron identidades",
     "display": {
       "reply_to": "Responder a:",

locales/fr/common.json

@@ -519,6 +519,7 @@
     "identity_created": "Identité créée avec succès",
     "identity_updated": "Identité mise à jour avec succès",
     "identity_deleted": "Identité supprimée",
+    "identity_set_primary": "Identité principale mise à jour",
     "identity_create_failed": "Échec de la création de l'identité: {error}",
     "identity_update_failed": "Échec de la mise à jour de l'identité: {error}",
     "identity_delete_failed": "Échec de la suppression de l'identité: {error}",
@@ -1367,6 +1368,7 @@
     "delete_confirm": "Supprimer cette identité ? Cette action est irréversible.",
     "cannot_delete": "Cette identité ne peut pas être supprimée",
     "primary_identity": "Principale",
+    "set_as_primary": "Définir comme principale",
     "no_identities": "Aucune identité trouvée",
     "display": {
       "reply_to": "Répondre à :",

locales/it/common.json

@@ -519,6 +519,7 @@
     "identity_created": "Identità creata con successo",
     "identity_updated": "Identità aggiornata con successo",
     "identity_deleted": "Identità eliminata",
+    "identity_set_primary": "Identità principale aggiornata",
     "identity_create_failed": "Impossibile creare l'identità: {error}",
     "identity_update_failed": "Impossibile aggiornare l'identità: {error}",
     "identity_delete_failed": "Impossibile eliminare l'identità: {error}",
@@ -1367,6 +1368,7 @@
     "delete_confirm": "Eliminare questa identità? Questa azione non può essere annullata.",
     "cannot_delete": "Questa identità non può essere eliminata",
     "primary_identity": "Principale",
+    "set_as_primary": "Imposta come principale",
     "no_identities": "Nessuna identità trovata",
     "display": {
       "reply_to": "Rispondi a:",

locales/ja/common.json

@@ -519,6 +519,7 @@
     "identity_created": "送信者情報を作成しました",
     "identity_updated": "送信者情報を更新しました",
     "identity_deleted": "送信者情報を削除しました",
+    "identity_set_primary": "プライマリ送信者情報を更新しました",
     "identity_create_failed": "送信者情報の作成に失敗しました: {error}",
     "identity_update_failed": "送信者情報の更新に失敗しました: {error}",
     "identity_delete_failed": "送信者情報の削除に失敗しました: {error}",
@@ -1367,6 +1368,7 @@
     "delete_confirm": "この送信者情報を削除しますか？この操作は元に戻せません。",
     "cannot_delete": "この送信者情報は削除できません",
     "primary_identity": "プライマリ",
+    "set_as_primary": "プライマリに設定",
     "no_identities": "送信者情報が見つかりません",
     "display": {
       "reply_to": "返信先:",

locales/nl/common.json

@@ -519,6 +519,7 @@
     "identity_created": "Identiteit succesvol aangemaakt",
     "identity_updated": "Identiteit succesvol bijgewerkt",
     "identity_deleted": "Identiteit verwijderd",
+    "identity_set_primary": "Primaire identiteit bijgewerkt",
     "identity_create_failed": "Kan identiteit niet aanmaken: {error}",
     "identity_update_failed": "Kan identiteit niet bijwerken: {error}",
     "identity_delete_failed": "Kan identiteit niet verwijderen: {error}",
@@ -1367,6 +1368,7 @@
     "delete_confirm": "Deze identiteit verwijderen? Dit kan niet ongedaan worden gemaakt.",
     "cannot_delete": "Deze identiteit kan niet worden verwijderd",
     "primary_identity": "Primair",
+    "set_as_primary": "Instellen als primair",
     "no_identities": "Geen identiteiten gevonden",
     "display": {
       "reply_to": "Antwoord naar:",

locales/pt/common.json

@@ -519,6 +519,7 @@
     "identity_created": "Identidade criada com sucesso",
     "identity_updated": "Identidade atualizada com sucesso",
     "identity_deleted": "Identidade excluída",
+    "identity_set_primary": "Identidade principal atualizada",
     "identity_create_failed": "Falha ao criar identidade: {error}",
     "identity_update_failed": "Falha ao atualizar identidade: {error}",
     "identity_delete_failed": "Falha ao excluir identidade: {error}",
@@ -1367,6 +1368,7 @@
     "delete_confirm": "Excluir esta identidade? Isso não pode ser desfeito.",
     "cannot_delete": "Esta identidade não pode ser excluída",
     "primary_identity": "Principal",
+    "set_as_primary": "Definir como principal",
     "no_identities": "Nenhuma identidade encontrada",
     "display": {
       "reply_to": "Responder para:",