feat(ext): static 'esc to cancel' subtitle in fullscreen form headers

All seven type forms plus the type-selection screen now show a small
'esc to cancel' subtitle under the heading when rendered in the
fullscreen vault tab (isInTab() === true). The subtitle is suppressed
in the popup, where esc has the more general meaning of closing the
popup. .form-subtitle class is shared between popup and vault
stylesheets so future hooks can reuse it.

Dynamic dirty-state ('unsaved · esc to cancel') wiring is deferred to
Phase 3 (unsaved-changes guard).

Plan 2026-04-30 fullscreen UX phase 1 task 8.

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

extension/src/popup/components/item-form.ts

@@ -54,12 +54,13 @@ export function renderItemForm(app: HTMLElement, mode: 'add' | 'edit'): void {
 function renderTypeSelection(app: HTMLElement): void {
   app.innerHTML = `
     <div class="pad">
-      <div style="display:flex; align-items:center; gap:12px; margin-bottom:16px;">
+      <div style="display:flex; align-items:center; gap:12px;">
         <button class="btn" id="back-btn">← back</button>
         <h3 style="margin:0;">new item</h3>
         <span style="flex:1;"></span>
         ${isInTab() ? '' : '<button class="btn" id="popout-btn" title="Open in tab">⤴</button>'}
       </div>
+      ${isInTab() ? '<div class="form-subtitle">esc to cancel</div>' : '<div style="margin-bottom:16px;"></div>'}
       <div class="type-select-list">
         ${TYPE_OPTIONS.map((opt) => `
           <button class="type-select-row" data-type="${opt.type}">