feat(ui): bundle Inter + design-system preview page (/app/design)

Slice 2: self-host Inter (variable) via @fontsource-variable/inter and add a
living design-system reference at /app/design showing tokens, the green ramp,
typography, and all primitives in both themes. Verified: frontend builds clean;
prod npm audit clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package-lock.json

@@ -8,6 +8,7 @@
       "name": "inboxintel-frontend",
       "version": "0.1.0",
       "dependencies": {
+        "@fontsource-variable/inter": "^5.2.8",
         "@radix-ui/react-avatar": "^1.2.1",
         "@radix-ui/react-checkbox": "^1.3.6",
         "@radix-ui/react-dialog": "^1.1.18",
@@ -764,6 +765,15 @@
       "integrity": "sha512-RiB/yIh78pcIxl6lLMG0CgBXAZ2Y0eVHqMPYugu+9U0AeT6YBeiJpf7lbdJNIugFP5SIjwNRgo4DhR1Qxi26Gg==",
       "license": "MIT"
     },
+    "node_modules/@fontsource-variable/inter": {
+      "version": "5.2.8",
+      "resolved": "https://registry.npmjs.org/@fontsource-variable/inter/-/inter-5.2.8.tgz",
+      "integrity": "sha512-kOfP2D+ykbcX/P3IFnokOhVRNoTozo5/JxhAIVYLpea/UBmCQ/YWPBfWIDuBImXX/15KH+eKh4xpEUyS2sQQGQ==",
+      "license": "OFL-1.1",
+      "funding": {
+        "url": "https://github.com/sponsors/ayuhito"
+      }
+    },
     "node_modules/@jridgewell/gen-mapping": {
       "version": "0.3.13",
       "resolved": "https://registry.npmjs.org/@jridgewell/gen-mapping/-/gen-mapping-0.3.13.tgz",