feat(playback): give the fullscreen side panel the settings panel's glass look

The fullscreen channel/episode panel was a full-height graphite strip while
the controls' settings panel is an inset, rounded glass card. The side panel
now wears the same surface: 16px inset (8px under 560px), 20px corners, the
controls' glass fill, hairline border, blur and shadow, the same open motion
and a 32px square close button. The edge hint uses the same glass, and the
episode list marks the playing row with the settings panel's selected cyan.

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

docs/architecture/player-controls-contract.md

@@ -767,9 +767,16 @@ ends. The panel carries the `dark-theme` context class, so app and Material
 tokens inside it resolve to the dark palette whatever the app theme is — and
 because the app's global `.dark-theme { background: … !important }` rule
 (`apps/web/src/styles.scss`) claims the background of every element wearing
-that class, the panel's translucent gradient is declared on the compound
+that class, the panel's glass fill is declared on the compound
 `.fullscreen-channel-panel.dark-theme` selector with `!important`; without
-that the panel painted no background at all.
+that the panel painted no background at all. The panel wears the same surface
+as the controls' settings panel — a card inset 16px from the player's top,
+bottom and left edges (8px in a player narrower than 560px), 20px corners,
+the `--pc-glass-bg` fill, hairline `--pc-glass-border`, the same blur and
+shadow, and a 32px square close button — with the values written literally,
+because the panel is a sibling of the controls host, outside the scope that
+declares the `--pc-*` palette. Its episode list marks the playing row with the
+settings panel's selected-option cyan tint.
 Keyboard: `C` is ignored while any editable element has focus and while the
 player sits inside an `inert` region; the search field is an ordinary input,
 so the controls' Space/K/F/M shortcuts stay out of it. That is also why `C` is