fix(ui): one watch-progress colour in app chrome and in the player

The same progressPercent was drawn red (#e50914) in the Up Next rail, the
fullscreen episode panel and the catalog capsule, blue (#4f8eff) in the
dock's timeline and Up next card, and the selection colour on the
dashboard, so one episode changed colour inside a single player overlay.

- App chrome reads a new --app-progress-color, declared per theme in
  m3-theme.scss as the theme's --app-selection-color: progress capsule,
  dashboard rail card and hero resume bar.
- Over video, watch progress reads --pc-progress from a shared
  _player-palette.scss partial. The controls host, the Up Next rail and
  the fullscreen episode panel (both outside the host's --pc-* scope)
  include its progress-token mixin.
- ArtPlayer's legacy skin gets PLAYER_PROGRESS_COLOR instead of #ff0000;
  a spec pins it to the Sass value.
- The rule is written into the UI guidelines' Progress Bars section,
  with specs in ui-playback, components and workspace-dashboard-feature.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/ui/components/src/lib/progress-capsule/progress-capsule.component.ts

@@ -34,11 +34,7 @@ import {
 
                 &__fill {
                     height: 100%;
-                    background: linear-gradient(
-                        90deg,
-                        #e50914 0%,
-                        #ff4d4d 100%
-                    );
+                    background: var(--app-progress-color);
                     transition: width 0.3s ease-out;
                     border-radius: 0 2px 2px 0;
                 }