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/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:",