Merge upstream/css-cleanup into master

Resolves conflicts between upstream/master and upstream/css-cleanup:
- CSS: take css-cleanup versions (button/link style unification, comment cleanup,
  table .button styles, [name=cancel]/.auxiliary styles)
- application_helper.rb: use SVG icon for flash message close button (css-cleanup)
- users_test.rb: use single-quote style (css-cleanup), keep sole-admin test (master)

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

app/assets/stylesheets/application.css

@@ -99,6 +99,13 @@ textarea {
   border-radius: 0.25em;
   padding: 0.2em 0.4em;
 }
+[name=cancel],
+.auxiliary {
+  border-color: var(--color-border-gray);
+  color: var(--color-nav-gray);
+  fill: var(--color-nav-gray);
+}
+input[type=checkbox],
 svg,
 textarea {
   margin: 0;
@@ -117,8 +124,8 @@ input[type=checkbox]:checked {
   appearance: checkbox;
   -webkit-appearance: checkbox;
 }
-/* Hide spin buttons of <input type=number>. */
-/* TODO: add spin buttons inside <input type=number>: before (-) and after (+) input. */
+/* Hide spin buttons in input number fields */
+/* TODO: add spin buttons inside input[number]: before (-) and after (+) input */
 input[type=number] {
   appearance: textfield;
   -moz-appearance: textfield;
@@ -348,6 +355,7 @@ header {
 }
 
 
+/* TODO: Hover over invalid should work like in measurements (thin vs thick border) */
 .labeled-form {
   align-items: center;
   display: grid;
@@ -384,7 +392,7 @@ header {
 .labeled-form .auxiliary {
   grid-column: 3;
   /* If more buttons are needed, `grid-row` can be replaced with
-   * `reading-flow: grid-columns` to ensure proper [tabindex] order. */
+   * `reading-flow: grid-columns` to ensure proper tabindex order */
   grid-row: 1;
   height: 100%;
   padding-block: 0;
@@ -546,6 +554,7 @@ table .button {
   color: var(--color-table-gray);
 }
 
+
 form table.items {
   border: none;
 }