feat: improve loading and error states on Batch and Analytics pages

Analytics page now shows skeleton loaders (cards and chart placeholders)
while data loads, and displays a retry button when the API call fails.
Batch page error state now shows the actual error message and suggests
user action.

Closes leeworks-agents/SPARC#16

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/Batch.tsx

@@ -114,9 +114,21 @@ export function Batch() {
 
       {/* Error */}
       {mutation.isError && (
-        <div className="flex items-center gap-2 bg-error/10 border border-error/20 text-error rounded-xl px-4 py-3">
-          <AlertCircle size={18} />
-          <span>Batch analysis failed. Please try again.</span>
+        <div className="bg-error/10 border border-error/20 rounded-xl px-4 py-3">
+          <div className="flex items-center gap-2 text-error">
+            <AlertCircle size={18} />
+            <span className="font-semibold">Batch analysis failed</span>
+          </div>
+          <p className="text-text-secondary text-sm mt-1 ml-7">
+            {mutation.error instanceof Error ? mutation.error.message : 'An unexpected error occurred.'}
+            {' '}Check your connection and try again.
+          </p>
+          <button
+            onClick={() => mutation.reset()}
+            className="ml-7 mt-2 text-sm text-primary hover:text-primary-dark underline"
+          >
+            Dismiss
+          </button>
         </div>
       )}