Add UI indicators for virtual parent accounts

Updates permission grant dialogs to visually distinguish virtual accounts:

Changes:
- Add custom option template to account selectors (both grant and bulk grant dialogs)
- Show "🌐 Virtual parent" caption explaining inheritance behavior
- Add blue "Virtual" chip badge to virtual accounts in dropdown
- Update hint text: "virtual accounts cascade to all children"
- Include is_virtual flag in accountOptions computed property

User Experience:
When admin selects account in grant dialog, virtual accounts now clearly show:
- "Expenses" with "Virtual" badge
- Caption: "grants access to all Expenses:* accounts"

This helps admins understand that granting permission on "Expenses" will
automatically give users access to all real expense accounts:
- Expenses:Groceries
- Expenses:Gas:Kitchen
- Expenses:Maintenance:Property
- etc.

Related: migrations.py m003 (created virtual parent accounts)

🤖 Generated with Claude Code (https://claude.com/claude-code)

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

static/js/permissions.js

@@ -67,7 +67,8 @@ window.app = Vue.createApp({
     accountOptions() {
       return this.accounts.map(acc => ({
         id: acc.id,
-        name: acc.name
+        name: acc.name,
+        is_virtual: acc.is_virtual || false
       }))
     },