A11y: Darken --color-accent-text to meet 4.5:1 contrast

The Linux E2E a11y test found the AI label in the search dialog failed
WCAG AA contrast (2.17:1 vs required 4.5:1). The `color-mix()` with 55%
black wasn't aggressive enough when the system accent color is light.
Bumped to 65% black so the variable meets its documented ≥4.5:1 contract
regardless of the system accent color.

Author: vdavid

apps/desktop/src/app.css

@@ -48,7 +48,7 @@
   --color-accent-text: color-mix(
     in srgb,
     var(--color-accent),
-    black 55%
+    black 65%
   ); /* Darkened accent for a11y when used as foreground text (≥4.5:1 on --color-bg-secondary) */
   --color-accent-hover: color-mix(in oklch, var(--color-accent), white 15%);
   --color-accent-subtle: color-mix(in oklch, var(--color-accent), transparent 85%);