fix(extension): drawer + lock-screen layout (center list, full-width lock)

- Closed vault drawer now negates its 440px via margin-right so it reserves no
  layout space — the list pane and its empty state center in the full
  post-sidebar width instead of being shifted left by the off-screen drawer.
- .vault-lock-screen gets flex:1 so the locked vault tab grows to full viewport
  width instead of collapsing to content width and pinning to the left.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pe8qw5KePDqAEBsAxnVQuJ

extension/src/vault/vault.css

@@ -1361,12 +1361,18 @@ textarea {
   background: var(--bg-elevated, #161b22);
   overflow-y: auto;
   transform: translateX(100%);
-  transition: transform 0.2s ease;
+  /* Closed: negate the 440px so the column reserves no layout space — the list
+     pane (and its empty state) then center in the full post-sidebar width
+     instead of being shifted left by the off-screen drawer. Opening restores
+     the width and slides the drawer in. */
+  margin-right: -440px;
+  transition: transform 0.2s ease, margin-right 0.2s ease;
   flex-shrink: 0;
 }
 
 .vault-drawer--open {
   transform: translateX(0);
+  margin-right: 0;
 }
 
 /* Empty state — centered, gold-accented icon, polished */
@@ -1689,6 +1695,10 @@ textarea {
 /* --- Lock screen (vault tab) --- */
 
 .vault-lock-screen {
+  /* #vault-app is a flex row (for the unlocked 3-column shell); when locked,
+     this is its sole child and must grow to the full viewport width or it
+     collapses to content width and pins to the left. */
+  flex: 1;
   display: flex;
   flex-direction: column;
   align-items: center;