theme auth/setup pages and unify resets

Phase 7: add LoadTheme to base :browser pipeline so auth and setup
pages get theme settings. Update root.html.heex with .themed wrapper,
font preloads, layer declaration, and generated CSS injection.
Remove old data-theme JS toggle script.

Phase 8: upgrade admin/reset.css to a proper @layer reset matching
the shop reset structure. Remove dead theme toggle CSS rules.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/admin/reset.css

@@ -1,20 +1,68 @@
-/* Minimal resets for admin pages */
+/* Minimal reset for admin and auth pages.
+   When shop.css is also loaded (admin pages), the shop reset takes precedence
+   since both sit in @layer reset. This ensures auth/setup pages still get
+   baseline resets when shop.css isn't present. */
 
-*, *::before, *::after {
-  box-sizing: border-box;
-}
+@layer reset {
+  *,
+  *::before,
+  *::after {
+    box-sizing: border-box;
+  }
 
-body {
-  margin: 0;
-  font-family: system-ui, -apple-system, sans-serif;
-}
+  * {
+    margin: 0;
+  }
 
-/* Make LiveView wrapper divs transparent for layout */
-[data-phx-session], [data-phx-teleported-src] {
-  display: contents;
-}
+  html {
+    height: 100%;
+    -webkit-text-size-adjust: 100%;
+    text-size-adjust: 100%;
+  }
 
-/* Phoenix LiveView loading states */
-.phx-no-feedback.phx-no-feedback {
-  /* Suppress validation styles until form is interacted with */
+  body {
+    height: 100%;
+    line-height: 1.5;
+    -webkit-font-smoothing: antialiased;
+  }
+
+  img,
+  picture,
+  video,
+  canvas,
+  svg {
+    display: block;
+    max-width: 100%;
+  }
+
+  input,
+  button,
+  textarea,
+  select {
+    font: inherit;
+    color: inherit;
+  }
+
+  a {
+    color: inherit;
+    text-decoration-skip-ink: auto;
+  }
+
+  button {
+    background: none;
+    border: none;
+    padding: 0;
+    cursor: pointer;
+  }
+
+  fieldset {
+    border: none;
+    padding: 0;
+  }
+
+  /* LiveView wrapper divs are layout-invisible */
+  [data-phx-session],
+  [data-phx-teleported-src] {
+    display: contents;
+  }
 }