add Tailwind and design tokens to frontend.

Tailwind v4 via Vite plugin, theme tokens in main.css, and VS Code hints for v4 at-rules.

.vscode/extensions.json

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+{
+    "recommendations": ["bradlc.vscode-tailwindcss"]
+}

.vscode/settings.json

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+{
+    "css.customData": [".vscode/tailwind.css-data.json"],
+    "files.associations": {
+        "**/frontend/src/assets/css/main.css": "tailwindcss"
+    },
+    "tailwindCSS.includeLanguages": {
+        "vue": "html"
+    },
+    "tailwindCSS.experimental.configFile": "frontend/src/assets/css/main.css"
+}

.vscode/tailwind.css-data.json

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+{
+    "version": 1.1,
+    "atDirectives": [
+        {
+            "name": "@theme",
+            "description": "Tailwind CSS v4 theme configuration."
+        },
+        {
+            "name": "@custom-variant",
+            "description": "Tailwind CSS v4 custom variant."
+        },
+        {
+            "name": "@apply",
+            "description": "Tailwind CSS utility composition."
+        },
+        {
+            "name": "@layer",
+            "description": "Tailwind CSS cascade layer."
+        },
+        {
+            "name": "@source",
+            "description": "Tailwind CSS v4 content source paths."
+        },
+        {
+            "name": "@utility",
+            "description": "Tailwind CSS v4 custom utility."
+        },
+        {
+            "name": "@variant",
+            "description": "Tailwind CSS v4 variant definition."
+        }
+    ]
+}