feat(ext/popup): hide popout-to-tab button in fullscreen forms

The ⤴ popout button is meaningless when the form is already in
vault.html — gate it on !isInTab(). Affects all seven type forms plus
the type-selection screen. Regression tests cover both popup (button
present) and fullscreen (button absent) contexts via it.each across
all 7 forms.

Plan 2026-04-30 fullscreen UX phase 1 task 7.

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

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

@@ -1,7 +1,7 @@
 /// Typed-item add/edit form dispatcher. Each type's renderForm lives in
 /// its own module under ./types/. Document stays "coming soon" until γ.
 
-import { navigate, getState, setState, escapeHtml, popOutToTab } from '../../shared/state';
+import { navigate, getState, setState, escapeHtml, popOutToTab, isInTab } from '../../shared/state';
 import type { Item, ItemType } from '../../shared/types';
 
 const TYPE_OPTIONS: Array<{ type: ItemType; icon: string; label: string }> = [
@@ -58,7 +58,7 @@ function renderTypeSelection(app: HTMLElement): void {
         <button class="btn" id="back-btn">← back</button>
         <h3 style="margin:0;">new item</h3>
         <span style="flex:1;"></span>
-        <button class="btn" id="popout-btn" title="Open in tab">⤴</button>
+        ${isInTab() ? '' : '<button class="btn" id="popout-btn" title="Open in tab">⤴</button>'}
       </div>
       <div class="type-select-list">
         ${TYPE_OPTIONS.map((opt) => `