Add basic monochrome popup UI with Tailwind CSS

Black-on-white, monospace, Universal Paperclips aesthetic.
All views: lock, setup/create/import, main account, send,
receive, add token, settings, and approval. Vanilla JS view
switching with stub state. README updated with full UI design
philosophy, external services documentation, and view descriptions.

package.json

@@ -13,8 +13,10 @@
         "fmt-check": "prettier --check ."
     },
     "devDependencies": {
+        "@tailwindcss/cli": "^4.2.1",
         "jest": "^30.2.0",
-        "prettier": "^3.8.1"
+        "prettier": "^3.8.1",
+        "tailwindcss": "^4.2.1"
     },
     "dependencies": {}
 }