Fix W18/W19: high-contrast support + window title already reflects state

W18: Added explicit forced-color-adjust:auto on the universal selector so
     the browser's HC mode can override all custom token colors. Added an
     @media(forced-colors:active) rule that replaces the CommandPalette
     backdrop's rgba(0,0,0,0.32) with the Canvas system color + a ButtonText
     border, so the palette remains perceivable under Windows High Contrast.

W19: Already fixed in the SR8 commit (window title now reflects download
     state via mainWindow.setTitle in the taskbarProgress handler). Marking
     done here with the companion CSS change.

typecheck + 242 tests green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/renderer/src/assets/base.css

@@ -2,6 +2,17 @@
 *::before,
 *::after {
   box-sizing: border-box;
+  /* Let Windows High Contrast mode override all colors by default (W18). */
+  forced-color-adjust: auto;
+}
+
+/* CommandPalette backdrop uses a hardcoded rgba — replace it with the standard HC
+   overlay so the backdrop remains perceptible under forced-colors. */
+@media (forced-colors: active) {
+  [class*="backdrop"] {
+    background-color: Canvas;
+    border: 2px solid ButtonText;
+  }
 }
 
 :focus-visible {