fix(web): the soundboard opens above the seated YouTube player

[janlauber]

Closes #2379

The seated jukebox player is z-[56] so it clears the stage it sits in. The soundboard's uncovered layer was z-[55], matched to the popped-out stage in #877 and never weighed against the seated player, and its default spot is the top-left of the main column — exactly where the lounge seats the video. So opening the board while watching something put it behind the player, grip included: nothing to press, nothing to drag.

stacking.test.ts already states the rule (#483): RMF forbids our chrome over the player, never a surface the rider pulled open, and "anything the rider opens clears the player". The board is opened and closed by the rider, like the people sheet, so it belongs above the seated player and below the drawers a rider opens on top of it.

• Soundboard.svelte: uncovered layer z-[57]; the covered drop to z-30 under modals is unchanged.
• stacking.test.ts: pins the board above the seated player and below the chat sheet, so the next renumbering notices.

🤖 Generated with Claude Code

[janlauber]

Verified in the running app (worktree dev server + Vite, mixer zeroed on all four channels, a YouTube entry queued over the room's socket, lounge at 1297×1234):

	board grip under the pointer	board layer	seated player layer
before (z-[55] forced inline)	the jukebox dock	55	56
after (this branch)	the soundboard	57	56

The board opens at 288,128 (364×365) and the player is seated at 273,82 (672×378), so the two overlap on every open of the lounge — with the old layer only the fader's sliver showed under the video's bottom edge, which is the report. elementFromPoint on the grip's centre is what flips between the rows above. No console errors; stacking.test.ts was red at 55 before the change and is green at 57.