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

lib/jmap/client.ts

@@ -1425,7 +1425,9 @@ export class JMAPClient {
       if (identityResponse.methodResponses?.[0]?.[0] === "Identity/get") {
         const identities = (identityResponse.methodResponses[0][1].list || []) as { id: string; email: string }[];
         if (identities.length > 0) {
-          const matchingIdentity = identities.find((id) => id.email === (fromEmail || this.username));
+          const target = fromEmail || this.username;
+          const matchingIdentity = identities.find((id) => id.email === target)
+            || (!target.includes('@') ? identities.find((id) => id.email.split('@')[0] === target) : undefined);
           finalIdentityId = matchingIdentity?.id || identities[0].id;
         }
       }