feat(profile): sign-in methods matrix — email add/change, provider link/unlink

Profile shows the account's sign-in methods for guests and durable accounts alike:
add or change email, link Telegram on the web (login widget), and unlink a linked
provider. Email is never unlinked (it is changed); unlink is offered only when
another identity remains, and a change to an address owned by another account shows
the non-disclosing 'check the address or contact support'. Add-VK-on-web stays
deferred (no VK OAuth).

Wires linkUnlink / changeEmailRequest / changeEmailConfirm through the client,
transport, mock and codec (encodeLinkUnlink + LinkResult 'unlinked'/'changed'
statuses); codec wire tests; ru/en i18n.

ui/src/lib/i18n/en.ts

@@ -179,6 +179,15 @@ export const en = {
   'profile.mergeBody': 'This identity already belongs to “{name}” ({games} games, {friends} friends).',
   'profile.mergeIrreversible': 'Merging combines both accounts into this one and cannot be undone.',
   'profile.mergeConfirm': 'Merge',
+  'profile.accountsTitle': 'Sign-in methods',
+  'profile.changeEmail': 'Change',
+  'profile.newEmailPlaceholder': 'New email address',
+  'profile.emailChanged': 'Email updated.',
+  'profile.emailChangeTaken': 'Check the address or contact support.',
+  'profile.unlink': 'Unlink',
+  'profile.unlinked': 'Account unlinked.',
+  'profile.unlinkTitle': 'Unlink account?',
+  'profile.unlinkBody': 'Remove {provider} as a sign-in method? You can link it again later.',
 
   'settings.title': 'Settings',
   'settings.theme': 'Theme',