fix(workspace): open a source row from the keyboard (#1885)

A Sources row opened only on a pointer click: the row div was not
focusable and had no key handler, so a keyboard user could Tab to its
actions but never open the playlist.

Following app-content-card, the row's title and meta block becomes the
keyboard activation element: role="button", tabindex="0", named by the
source title, aria-current on the active source, aria-disabled while
busy. Enter and Space open the source (Space prevents the page scroll).
The drag handle, health indicator and actions stay its siblings, so no
control is nested inside a role="button" and their keys never bubble
into it. A click anywhere on the row still opens it via the row.

The row draws the ring with the shared focus-ring mixin, inset, because
the list's scroller would cut a ring drawn outside the full-width row.
playlist-shared-ui now declares its ui-styles stylesheet dependency.

Tests: unit spec for Enter/Space, bubbling from an action button, busy
and selected states; Electron E2E tabs to a source row, checks the ring
and opens it with Enter (fails on the previous component).

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.changes/workspace-sources-keyboard-open.md

@@ -0,0 +1,8 @@
+---
+type: fix
+area: workspace
+---
+
+Sources can now be opened from the keyboard: on the Sources page, Tab stops
+on each source row before its buttons, and Enter or Space opens it just like
+a click. The focused row shows the focus ring.