feat(ext/vault): add brand logo to lock screen

Mirrors the logo-lockup treatment already used in the popup unlock view
(Phase 2B) and the setup wizard. Lock-screen rendering now shows the
relicario-logo.svg above the wordmark instead of just the wordmark.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

extension/src/vault/vault.ts

@@ -270,6 +270,7 @@ function render(): void {
 function renderLockScreen(app: HTMLElement): void {
   app.innerHTML = `
     <div class="vault-lock-screen">
+      <img class="brand-logo" src="icons/relicario-logo.svg" alt="">
       <span class="brand">Relicario</span>
       <div class="vault-lock-screen__form">
         <input type="password" id="vault-passphrase" placeholder="passphrase" autocomplete="off" />