feat: add admin ideas management page with private notes

Adds a dedicated super admin page for managing idea submissions across
all tenants. Includes status summary cards, filterable/searchable table,
detail modal with status updates, and private admin notes for internal
tracking (sprint refs, thoughts, follow-ups). Notes are not visible to
tenant users.

- Database: admin_note column on shared.ideas (019 migration)
- Backend: PUT /admin/ideas/:id/note endpoint
- Frontend: AdminIdeasPage with table, filters, detail modal
- Sidebar: "Idea Submissions" nav link in admin sections
- Routing: /admin/ideas route under SuperAdminRoute guard

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -29,6 +29,7 @@ import { SettingsPage } from './pages/settings/SettingsPage';
 import { UserPreferencesPage } from './pages/preferences/UserPreferencesPage';
 import { OrgMembersPage } from './pages/org-members/OrgMembersPage';
 import { AdminPage } from './pages/admin/AdminPage';
+import { AdminIdeasPage } from './pages/admin/AdminIdeasPage';
 import { AssessmentGroupsPage } from './pages/assessment-groups/AssessmentGroupsPage';
 import { CashFlowForecastPage } from './pages/cash-flow/CashFlowForecastPage';
 import { MonthlyActualsPage } from './pages/monthly-actuals/MonthlyActualsPage';
@@ -133,6 +134,7 @@ export function App() {
         }
       >
         <Route index element={<AdminPage />} />
+        <Route path="ideas" element={<AdminIdeasPage />} />
       </Route>
 
       {/* Main app routes (require auth + org) */}