feat(FN-3104): switch todos to modal-only navigation

- Add TodoModal component and styling, and mount it through AppModals
- Remove dedicated Todos view routing and drive todos access through modal state/actions
- Update Header and MobileNavBar interactions plus modal manager/view-state hooks for modal flow
- Expand dashboard tests to cover modal rendering, open/close behavior, and updated app/header expectations

Fusion-Task-Id: FN-3104

packages/dashboard/app/components/TodoView.css

@@ -6,34 +6,6 @@
   overflow: hidden;
 }
 
-.todo-view-header {
-  display: flex;
-  flex-direction: row;
-  justify-content: space-between;
-  align-items: center;
-  padding: var(--space-lg) var(--space-xl);
-  border-bottom: 1px solid var(--border);
-  background: var(--surface);
-  gap: var(--space-md);
-  flex-shrink: 0;
-}
-
-.todo-view-header h2 {
-  font-size: 1rem;
-  font-weight: 600;
-  color: var(--text);
-  margin: 0;
-  display: flex;
-  align-items: center;
-  gap: var(--space-sm);
-}
-
-.todo-view-description {
-  color: var(--text-muted);
-  font-size: 0.8125rem;
-  margin: 2px 0 0 0;
-}
-
 .todo-view-layout {
   display: flex;
   flex-direction: row;