fix: Chrome draws the popup in its monospace font (closes #418)

Chrome adds a stylesheet of its own to extension pages that sets the font
on body. Tailwind 4 puts its classes in a cascade layer, and a rule outside
any layer wins over them, so font-mono lost and Chrome drew the popup in
the system font. body now carries font-mono!, which marks the class
important. Both end-to-end suites check the popup's font.

The same stylesheet also makes Chrome draw the popup's text at 12px rather
than text-sm's 14px; that is unchanged here and filed as issue 456.

Model: opus-5-5

src/popup/index.html

@@ -6,7 +6,10 @@
         <title>AutistMask</title>
         <link rel="stylesheet" href="styles.css" />
     </head>
-    <body class="bg-bg text-fg font-mono text-sm">
+    <!-- Chrome gives extension pages a stylesheet of its own that sets the
+         font on body, and a Tailwind class beats it only when marked
+         important: hence font-mono! rather than font-mono. -->
+    <body class="bg-bg text-fg font-mono! text-sm">
         <div id="app" class="p-2 pr-5 overflow-x-hidden">
             <!-- ============ GLOBAL TITLE BAR ============ -->
             <div