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.

src/popup/styles/main.css

@@ -1,18 +1,18 @@
-* {
-    margin: 0;
-    padding: 0;
-    box-sizing: border-box;
+@import "tailwindcss";
+
+@theme {
+    --font-mono:
+        ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas,
+        "Liberation Mono", monospace;
+    --color-bg: #ffffff;
+    --color-fg: #000000;
+    --color-muted: #666666;
+    --color-border: #000000;
+    --color-border-light: #cccccc;
+    --color-hover: #eeeeee;
 }
 
 body {
     width: 360px;
     min-height: 600px;
-    font-family:
-        -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
-    background: #1a1a2e;
-    color: #e0e0e0;
-}
-
-#app {
-    padding: 16px;
 }