Fix prose-invert to only apply in dark mode on Analysis page

The prose-invert class was applied unconditionally, causing inverted
(light) text in light mode within the AI analysis results section.
Changed to dark:prose-invert so it only activates when dark mode is
enabled.

Note: The broader dark mode feature (issue #1605) is already fully
implemented -- ThemeContext, toggle button, CSS variables, dark:
variants across all pages. This fix addresses the only remaining
unstyled element.

Closes leeworks-agents/SPARC#1605

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

frontend/src/pages/Analysis.tsx

@@ -159,7 +159,7 @@ export function Analysis() {
                   </button>
                 </div>
               </div>
-              <div className="prose prose-invert max-w-none">
+              <div className="prose dark:prose-invert max-w-none">
                 <div className="text-text-primary whitespace-pre-wrap leading-relaxed">
                   {result.analysis}
                 </div>