fix(dashboard): scroll a focused rail card fully into view

Chromium skips its focus scroll when 32px or more of the element already
shows, so Tab onto the last source card of a rail that overflows by less
than a card left it half-hidden under the edge fade. The rail track now
handles focusin and scrolls to the first card-start snap position that
reveals the whole card; a plain "nearest" scroll is not enough because
mandatory snapping can round it back (seen on the live channel rail).

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

.changes/dashboard-rail-focus-reveal.md

@@ -0,0 +1,8 @@
+---
+type: fix
+area: dashboard
+---
+
+Moving keyboard focus onto a dashboard rail card, such as a source in Recently
+Used Sources, now scrolls the rail so the whole card shows instead of leaving
+it half-hidden under the edge fade.