fix(playback): make the fullscreen channel panel discoverable and calmer to close (#1619)

* fix(playback): make the fullscreen channel panel discoverable and calmer to close

The left-edge hot zone was an invisible 28px strip with nothing telling the
user where the list lived, so "hovering the left side" rarely reached it while
`C` always worked. Mouse movement over the stage now reveals a slim edge hint
tab (CSS chevron, fades after 2.5 s idle, lit while the pointer rests in the
zone), the zone grows to 40px (48px coarse), and a click or tap on the edge
opens at once without the dwell.

Closing follows the pointer more honestly: the mouse-leave grace grows from
420 ms to 1 s, and it applies only once the pointer has engaged with the
panel, so a `C`-opened list no longer closes while the mouse merely roams over
the video under the user's typing. Clicks inside the panel never close it.

Found on the way: closing with Escape while the mouse still rests on the edge
reopened the panel 160 ms later, because Chromium synthesizes a `pointerenter`
on the hot zone the aside slid away from. An explicit close now re-arms the
zone only on the next real pointer move.

Verified live over CDP on HTML5, Video.js, ArtPlayer and Embedded MPV
frame-copy; the hover mechanism itself was sound on every engine.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

* fix(playback): open the panel only on a primary click begun on the edge

Review follow-up. The hot zone opened on every pointerup, so a drag released
over the edge, a right or middle click and a pen barrel button all opened the
panel; now pointerdown records the primary pointer and pointerup must match
it (a leave or cancel forgets the press). The synthetic pointerenter that
follows an explicit close no longer arms the hint either: the edge stays
clear until the next real pointer move, which arms it and starts the dwell.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

.changes/playback-fullscreen-panel-edge-hint.md

@@ -0,0 +1,6 @@
+---
+type: fix
+area: playback
+---
+
+The fullscreen channel list is easier to find and calmer to use: moving the mouse shows a small tab on the left edge, a click on that edge opens the list at once, hover still opens it after a short rest, and the list now stays open for a full second after the mouse leaves. Opened with `C`, it no longer closes while the mouse only wanders over the video.