fix(theme): make scrollbars follow the app theme instead of the OS color scheme

On Windows with a light OS theme, scrollbars rendered light even when the
app was switched to dark. Two combined causes:

- The page never declared `color-scheme`, so Chromium colored native
  scrollbars from the OS preference. Declare `color-scheme: light` on html
  and flip it to `dark` via `html:has(> body.dark-theme)` plus the
  `.dark-theme` block itself.
- Scrollbar styling referenced `--mat-sys-*` tokens, which are never
  emitted by the current Material theme setup (mat.define-theme +
  all-component-themes does not produce system tokens). Those
  `scrollbar-color` declarations computed to `auto`, falling back to the
  native (light) scrollbar. Switch scrollbar styling to the `--app-muted-color`
  design token (defined for both themes), replace hardcoded white
  `rgba(255,255,255,.08)` thumbs, and add an explicit `scrollbar-color`
  where only `scrollbar-width: thin` was set.

Verified live in Electron via CDP in both themes: scrollbar-color resolves
and scrollbars render dark in dark theme regardless of the OS setting.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

libs/workspace/shell/feature/src/lib/workspace-command-palette/workspace-command-palette.component.scss

@@ -89,7 +89,8 @@
     overflow-x: hidden;
     padding: 6px;
     scrollbar-width: thin;
-    scrollbar-color: var(--mat-sys-outline-variant) transparent;
+    scrollbar-color: color-mix(in srgb, var(--app-muted-color) 55%, transparent)
+        transparent;
 }
 
 // ─── Group ───────────────────────────────────────────────────────────────────

libs/workspace/shell/feature/src/lib/workspace-context-panel/workspace-context-panel.component.scss

@@ -177,7 +177,8 @@ app-workspace-context-category-view {
     overflow-x: hidden;
     overscroll-behavior: contain;
     scrollbar-width: thin;
-    scrollbar-color: var(--mat-sys-outline-variant) transparent;
+    scrollbar-color: color-mix(in srgb, var(--app-muted-color) 55%, transparent)
+        transparent;
 }
 
 .context-skeleton {