refactor(frontend): enforce semantic tokens + extract 3 Screen Models

Color tokens: 3658→0 violations across 186 .svelte/.svelte.ts files.
All raw Tailwind colors (bg-blue-*, text-gray-*, border-red-*, etc.)
replaced with semantic tokens from tailwind.config.js in 5 perl passes.

Model extraction (11→8 oversized pages):
- LLMReportModel.svelte.ts: LLM report page 413→221 lines
- DatasetDetailModel.svelte.ts: dataset detail page 416→218 lines
- DatasetsHubModel.svelte.ts: datasets hub page 468→246 lines

Tests: 14 assertions updated (color classes + model refs).
Build: 1 duplicate class:text-primary fix in ValidationTaskForm.
All 699/699 tests pass.

frontend/src/lib/ui/HelpTooltip.svelte

@@ -30,7 +30,7 @@
     stroke-width="2"
     stroke-linecap="round"
     stroke-linejoin="round"
-    class="text-gray-400 hover:text-blue-500 cursor-help transition-colors shrink-0"
+    class="text-text-subtle hover:text-primary cursor-help transition-colors shrink-0"
     aria-hidden="true"
   >
     <circle cx="12" cy="12" r="10" />
@@ -45,8 +45,8 @@
       absolute bottom-full left-1/2 -translate-x-1/2 mb-2
       w-64 sm:w-72
       z-50
-      rounded-lg border border-gray-200 bg-white px-3 py-2
-      text-xs text-gray-700 shadow-lg
+      rounded-lg border border-border bg-surface-card px-3 py-2
+      text-xs text-text shadow-lg
       opacity-0 group-hover:opacity-100 group-focus:opacity-100
       transition-opacity duration-150
       pointer-events-none