fix(embedded-mpv): make slider track and thumb visible

The timeline and volume sliders were rendering invisibly because:

- --embedded-mpv-accent resolved to var(--mat-sys-primary) with no
  fallback. If the Material 3 theme tokens did not cascade into the
  component scope, the variable was empty and both the thumb and the
  Firefox progress fill rendered with no color.
- The track contrast was 22% of var(--mat-sys-on-surface) which has the
  same scoping fragility on top of being subtle even when it does
  resolve.
- The thumb had no border, so even when accent was present it could
  blend into similarly-toned popover backgrounds.
- The played portion of the timeline had no fill in Chromium because
  ::-webkit-slider-runnable-track does not support a native progress
  state.

Fixes:

- Add hard fallbacks on every theme-token reference (accent falls back to
  #7dd3fc, surface to #0f1620, outline-variant to a translucent white).
- Add --embedded-mpv-track and --embedded-mpv-thumb-ring as theme-
  independent CSS variables so the slider stays legible regardless of
  the parent theme provider.
- Give the thumb a 2 px white ring border + a stronger drop shadow so it
  reads on any background (panel glass, dark backdrop, accent-coloured
  area).
- Implement played-progress fill via a linear-gradient on
  ::-webkit-slider-runnable-track keyed off a CSS variable
  --slider-progress, set per-slider from the template based on the
  current playback position / volume. Firefox uses native
  ::-moz-range-progress.
- Tidy disabled state styling so live-stream timelines (no duration)
  are dim but still visible.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Entire-Checkpoint: f957cd9849e0

libs/ui/playback/src/lib/embedded-mpv-player/embedded-mpv-player.component.html

@@ -128,6 +128,13 @@
             <div class="embedded-mpv-player__timeline">
                 <input
                     class="embedded-mpv-player__slider"
+                    [style.--slider-progress.%]="
+                        canSeek()
+                            ? (timelineValue() /
+                                  ((session()?.durationSeconds ?? 0) || 1)) *
+                              100
+                            : 0
+                    "
                     type="range"
                     min="0"
                     [max]="session()?.durationSeconds ?? 0"
@@ -185,6 +192,7 @@
                         >
                             <input
                                 class="embedded-mpv-player__slider embedded-mpv-player__slider--volume"
+                                [style.--slider-progress.%]="volume() * 100"
                                 type="range"
                                 min="0"
                                 max="1"

libs/ui/playback/src/lib/embedded-mpv-player/embedded-mpv-player.component.scss

@@ -4,13 +4,18 @@
 }
 
 .embedded-mpv-player {
-    --embedded-mpv-accent: var(--mat-sys-primary);
+    --embedded-mpv-accent: var(--mat-sys-primary, #7dd3fc);
+    --embedded-mpv-track: rgba(255, 255, 255, 0.28);
+    --embedded-mpv-thumb-ring: rgba(255, 255, 255, 0.95);
     --embedded-mpv-glass: color-mix(
         in srgb,
-        var(--mat-sys-surface) 78%,
+        var(--mat-sys-surface, #0f1620) 78%,
         transparent
     );
-    --embedded-mpv-border: var(--mat-sys-outline-variant);
+    --embedded-mpv-border: var(
+        --mat-sys-outline-variant,
+        rgba(255, 255, 255, 0.16)
+    );
     --embedded-mpv-controls-height: 64px;
 
     position: relative;
@@ -252,9 +257,11 @@
 }
 
 .embedded-mpv-player__slider {
+    --slider-progress: 0%;
     width: 100%;
     height: 18px;
     appearance: none;
+    -webkit-appearance: none;
     background: transparent;
     cursor: pointer;
 }
@@ -265,26 +272,27 @@
     border-radius: 4px;
 }
 
-/* WebKit / Chromium */
+/* WebKit / Chromium — track shows played portion via linear-gradient. */
 .embedded-mpv-player__slider::-webkit-slider-runnable-track {
     height: 4px;
     border-radius: 999px;
-    background: color-mix(
-        in srgb,
-        var(--mat-sys-on-surface) 22%,
-        transparent
+    background: linear-gradient(
+        to right,
+        var(--embedded-mpv-accent) var(--slider-progress),
+        var(--embedded-mpv-track) var(--slider-progress)
     );
 }
 
 .embedded-mpv-player__slider::-webkit-slider-thumb {
+    -webkit-appearance: none;
     appearance: none;
     width: 14px;
     height: 14px;
     margin-top: -5px;
-    border: 0;
+    border: 2px solid var(--embedded-mpv-thumb-ring);
     border-radius: 50%;
     background: var(--embedded-mpv-accent);
-    box-shadow: 0 1px 4px rgba(0, 0, 0, 0.4);
+    box-shadow: 0 1px 6px rgba(0, 0, 0, 0.55);
     transition: transform 120ms ease-out;
 }
 
@@ -293,33 +301,36 @@
     transform: scale(1.18);
 }
 
+.embedded-mpv-player__slider:disabled::-webkit-slider-runnable-track {
+    background: var(--embedded-mpv-track);
+}
+
 .embedded-mpv-player__slider:disabled::-webkit-slider-thumb {
-    background: color-mix(
-        in srgb,
-        var(--mat-sys-on-surface) 32%,
-        transparent
-    );
+    background: rgba(255, 255, 255, 0.45);
+    border-color: rgba(255, 255, 255, 0.6);
     box-shadow: none;
 }
 
-/* Firefox */
+/* Firefox — native ::-moz-range-progress for played portion. */
 .embedded-mpv-player__slider::-moz-range-track {
     height: 4px;
     border-radius: 999px;
-    background: color-mix(
-        in srgb,
-        var(--mat-sys-on-surface) 22%,
-        transparent
-    );
+    background: var(--embedded-mpv-track);
+}
+
+.embedded-mpv-player__slider::-moz-range-progress {
+    height: 4px;
+    border-radius: 999px;
+    background: var(--embedded-mpv-accent);
 }
 
 .embedded-mpv-player__slider::-moz-range-thumb {
     width: 14px;
     height: 14px;
-    border: 0;
+    border: 2px solid var(--embedded-mpv-thumb-ring);
     border-radius: 50%;
     background: var(--embedded-mpv-accent);
-    box-shadow: 0 1px 4px rgba(0, 0, 0, 0.4);
+    box-shadow: 0 1px 6px rgba(0, 0, 0, 0.55);
     transition: transform 120ms ease-out;
 }
 
@@ -328,6 +339,12 @@
     transform: scale(1.18);
 }
 
+.embedded-mpv-player__slider:disabled::-moz-range-thumb {
+    background: rgba(255, 255, 255, 0.45);
+    border-color: rgba(255, 255, 255, 0.6);
+    box-shadow: none;
+}
+
 .embedded-mpv-player__control-panel {
     min-width: 0;
     color: var(--mat-sys-on-surface);