feat(ext/vault): sidebar logo before "Relicario" wordmark

Renders the 16-optimized SVG (icons/relicario-logo-16.svg) inline
before the brand text in .vault-sidebar__header. Sized to 20×20 px
with flex-shrink: 0 so it survives narrow-pane wraps. The header
already had display: flex + gap: 8px, so the layout absorbed the new
element without further changes. Popup surface is untouched (this
override is scoped to .vault-sidebar__header .brand-logo).

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

extension/src/vault/vault.ts

@@ -258,6 +258,7 @@ function renderShell(app: HTMLElement): void {
     app.innerHTML = `
       <div class="vault-sidebar">
         <div class="vault-sidebar__header">
+          <img class="brand-logo" src="icons/relicario-logo-16.svg" alt="">
           <span class="brand">Relicario</span>
         </div>
         <div class="vault-sidebar__search">