refactor: consolidate loading/error states and extract instance actions hook

Frontend:
- Create reusable DataStates components (LoadingState, ErrorState, EmptyState)
- Refactor 12 pages to use shared state components instead of inline JSX
- Extract useInstanceActions hook to eliminate session action duplication
- Update dashboard and sessions pages to use shared hook

OpenSpec:
- Archive completed mobile-app-usability change (44/44 tasks)
- Archive completed add-config-profiles change (15/15 tasks)

Quality: TypeScript check passes, production build succeeds

apps/web/src/components/data-states.tsx

@@ -0,0 +1,34 @@
+import { Icon } from "./icon";
+
+interface LoadingStateProps {
+  message?: string;
+}
+
+export const LoadingState = ({ message = "Loading..." }: LoadingStateProps) => (
+  <p className="muted">{message}</p>
+);
+
+interface ErrorStateProps {
+  message?: string;
+  onRetry?: () => void;
+}
+
+export const ErrorState = ({ message = "Failed to load", onRetry }: ErrorStateProps) => (
+  <div className="card stack">
+    <p>{message}</p>
+    {onRetry && (
+      <button className="secondary-button" onClick={onRetry} type="button">
+        <Icon name="refresh" size="sm" />
+        Retry
+      </button>
+    )}
+  </div>
+);
+
+interface EmptyStateProps {
+  message: string;
+}
+
+export const EmptyState = ({ message }: EmptyStateProps) => (
+  <p className="muted">{message}</p>
+);