refactor(frontend): Task 6 — Modularize utility functions by domain

New utility modules in utils/:
- messageUtils.ts: createMessage() factory, formatContentForClipboard()
- contentParsing.ts: parseTextContent(), resolveImageUrl(), buildGeneratedContent()
- sseParser.ts: parseSSEStream() — eliminates duplicated SSE decode loop
- generationStages.ts: getGenerationStage() — pure progress-stage mapper
- briefFields.ts: BRIEF_FIELD_LABELS, BRIEF_DISPLAY_ORDER, BRIEF_FIELD_KEYS
- stringUtils.ts: escapeRegex(), createNameSwapper(), matchesAnyKeyword()
- apiUtils.ts: retryRequest (exponential backoff), RequestCache, throttle
- index.ts: barrel export for all utils

Deduplicated code:
- msg() helper was identical in 2 hooks → createMessage() in messageUtils
- SSE stream parser was identical in 2 API functions → parseSSEStream()
- GeneratedContent parsing was near-identical in 3 hooks → buildGeneratedContent()
- Brief field constants duplicated in 2 components → shared briefFields
- Keyword matching pattern repeated 3x → matchesAnyKeyword()

Internal helpers marked non-exported:
- rewriteBlobUrl() in contentParsing.ts
- defaultShouldRetry(), sleep() in apiUtils.ts
- plainDownload() in downloadImage.ts (already was)

Build: 0 TypeScript errors

Author: Shubhangi-Microsoft

.coverage

@@ -12,9 +12,11 @@
   "dependencies": {
     "@fluentui/react-components": "^9.54.0",
     "@fluentui/react-icons": "^2.0.245",
+    "@reduxjs/toolkit": "^2.11.2",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-markdown": "^9.0.1",
+    "react-redux": "^9.2.0",
     "uuid": "^10.0.0"
   },
   "devDependencies": {