feat(validation): chunk screenshots by max_images limit + fix websocket crash

- analyze_dashboard_multimodal now splits screenshots into chunks
  of max_images (from provider config) and sends them in parallel
- Results merged: worst status, deduped issues by (severity, msg, loc)
- New helper methods: _deduplicate_issues, _merge_chunk_results, _call_llm_for_images
- Plugin passes db_provider.max_images to the LLM client
- Report UI shows 'Chunked ×N' badge when analysis used multiple chunks
- i18n: added 'chunked' / 'По частям' key to validation.json
- Fix: isinstance(StopIteration) -> isinstance(_ws_exc, StopIteration)
  which crashed the websocket and broke task execution mid-flight
- Fix: update test mocks (_FakeLLMClient, _FakeScreenshotService)

frontend/src/lib/i18n/locales/en/validation.json

@@ -212,5 +212,6 @@
   "path_b": "Path B — Text-only",
   "no_issues": "No issues detected",
   "no_logs": "No logs available",
-  "no_screenshots": "No screenshots saved"
+  "no_screenshots": "No screenshots saved",
+  "chunked": "Chunked"
 }

frontend/src/lib/i18n/locales/ru/validation.json

@@ -212,5 +212,6 @@
   "path_b": "Путь B — Только текст",
   "no_issues": "Проблем не обнаружено",
   "no_logs": "Нет доступных логов",
-  "no_screenshots": "Нет сохраненных скриншотов"
+  "no_screenshots": "Нет сохраненных скриншотов",
+  "chunked": "По частям"
 }

frontend/src/routes/validation-tasks/[policyId]/runs/[runId]/+page.svelte

@@ -414,6 +414,11 @@
 							{#if dbPath === 'A'}
 								<span class="text-purple-600 font-medium">📸 {$t.validation?.path_a || 'Path A — Screenshot'}</span>
 								<span class="text-gray-400">| {$t.validation?.screenshots || 'Screenshots'} captured via browser automation</span>
+								{#if dashboard.chunk_count && dashboard.chunk_count > 1}
+									<span class="inline-flex items-center gap-1 px-2 py-0.5 rounded-full bg-amber-100 text-amber-700 text-xs font-medium">
+										{$t.validation?.chunked || 'Chunked'} ×{dashboard.chunk_count}
+									</span>
+								{/if}
 							{:else}
 								<span class="text-teal-600 font-medium">📝 {$t.validation?.path_b || 'Path B — Text-only'}</span>
 								<span class="text-gray-400">| {$t.validation?.issues || 'Issue'} detection via DOM extraction</span>