fix: gate gray-500 muted text contrast behind High Contrast Mode (#27556)

Follow-up to #27497, reopened as a high contrast mode change, and the third and last of the contrast set after #27554 and #27555.

The grey scale in src/tailwind.css is achromatic oklch(L 0 0), so relative luminance is exactly L³ and text-gray-500 is 2.77:1 on white, against the 4.5:1 required by WCAG 1.4.3 and the 3:1 required of icons by 1.4.11. Around 290 sites use text-gray-500 dark:text-gray-400 for secondary labels, descriptions, counters and icons. Dark mode already passes at 6.46:1 and is left alone.

Rather than rewriting the class literal at every site, the remap is two CSS rules that only apply when the existing High Contrast Mode setting is on, so the default theme is untouched. Light mode resolves to gray-600 (5.75:1).

The split is not cosmetic. The `text-gray-500` utility is overridden inside `@layer utilities` with `:where()` so the rule sits below `hover:text-gray-*` and `dark:hover:text-gray-*` in specificity and hover feedback keeps working. The `.app-muted`, `.app-icon-muted` and `.tiptap table` classes in src/app.css are declared unlayered, which means no layered rule can reach them, so their override is unlayered too. They are `@apply text-gray-500 dark:text-gray-400` and fail identically, so leaving them out would have left the slash command menu and tiptap tables below 4.5:1 with the setting on.

Verified in a browser against Tailwind's emitted rules and layer order: with the setting on, the utility, .app-muted and .tiptap table all resolve to gray-600 in light while a focusable element carrying hover:text-gray-700 still resolves to gray-700 on interaction; dark mode and the setting-off case are unchanged in both themes.

One site is deliberately left out: EmbeddedChatHistoryDropdown.svelte uses text-gray-500/70, a separate class token that the selector does not match.

src/app.css

@@ -854,3 +854,16 @@ body {
 		color: var(--color-gray-500);
 	}
 }
+
+/* High Contrast Mode: gray-500 muted text is 2.77:1 in light (WCAG 1.4.3 wants 4.5:1).
+   The utility stays in the layer so hover variants still outrank it; the classes below
+   are unlayered, so their override has to be too. */
+@layer utilities {
+	html:where(.high-contrast):not(:where(.dark)) .text-gray-500 {
+		color: var(--color-gray-600);
+	}
+}
+
+html.high-contrast:not(.dark) :is(.app-muted, .app-icon-muted, .tiptap table) {
+	color: var(--color-gray-600);
+}