Make the border reset additive: zero default width

Previous reset set border-style: solid globally but kept the
browser-default border-width, which surfaced 1-2px outlines on
every <button>, <input>, <header> etc. Match the relevant slice of
Tailwind's Preflight by also resetting border-width to 0, so only
elements that explicitly opt in via a border utility (or a legacy
shorthand rule like `border: 2px solid #ddd`) draw a border.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

public/index.html

@@ -86,11 +86,17 @@
         .material-symbols-outlined {
             font-variation-settings: 'FILL' 0, 'wght' 400, 'GRAD' 0, 'opsz' 24;
         }
-        /* Tailwind preflight is disabled to keep legacy screens working;
-           manually restore the border defaults Tailwind utilities expect. */
+        /* Tailwind preflight is disabled to keep legacy screens working,
+           but the border utilities expect Preflight's defaults. Restore
+           just the border slice: reset width to 0 and set a default
+           style+colour so Tailwind's `border` / `border-b` etc. work as
+           expected, while not turning every browser-default control
+           outline into a visible 2px border. Legacy CSS rules that set
+           shorthand `border: …` still win via their class specificity. */
         *, ::before, ::after {
-            border-color: #c3c6d7;
+            border-width: 0;
             border-style: solid;
+            border-color: #c3c6d7;
         }
     </style>