fix(web): stop paneOverlay ref-count effect self-looping on mobile (BUG-2284)

[xarmian]

What

Fixes the red E2E on main — two failures introduced by #1007 (TASK-2131, "full ARIA-modal isolation for the mobile detail pane"), both stemming from making the mobile pane role="dialog".

1. The real defect — mobile pane renders empty (effect_update_depth_exceeded)

#1007 added a PaneHost $effect that calls paneOverlay.enter()/leave() to inert the app-shell chrome behind the mobile overlay. enter()'s overlayCount += 1 reads overlayCount inside that tracked effect scope, so the effect took a reactive dependency on the very signal it writes:

effect runs → enter() writes overlayCount → effect dirty → re-run → enter() → … → effect_update_depth_exceeded

Svelte aborts the flush, which strands the rest of the subtree's reactivity: paneMintForRoute (in the collection page) stopped recomputing, so {#if paneMintForRoute} stayed false and the mobile pane rendered completely empty — no item content, no Back chevron. #1007's manual Playwright check verified the ARIA attributes but not that item content still rendered.

Fix: untrack the count read in enter()/leave() so a write from an effect never establishes a self-dependency. The write still notifies the layout's mobileOverlayActive reader normally. The ref-count mutators are written from effects by design, so the untrack belongs in the store (matches the existing untrack-the-write idiom in UserSettingsForm.svelte / Lightbox.svelte).

2. Test-locator collision

The pane is now role="dialog" on mobile, so pane-controller.spec.ts:771's [role="dialog"] + text locator matched both the pane (which contains the sheet) and the BottomSheet → strict-mode violation. Target the sheet by accessible name (getByRole('dialog', { name: 'Quick actions' })) — the pane's is "Item detail".

Diagnosis

Bisected the red E2E to #1007 (parent 316e25a6 green, b76abdd6 red). Instrumented the collection page + PaneHost to trace the nav/mint sequence and captured the effect_update_depth_exceeded pageerror; confirmed by isolation that disabling the overlay effect removed the loop, and that restoring it with untrack keeps #1007's a11y isolation working.

Verification

• cd web && npm run check — svelte-check 0 errors
• paneOverlay.svelte.test.ts — 4 pass (ref-count semantics)
• pane-controller (771 + 1178) now pass; pane-controller + pane-a11y-focus + pane-mobile-overlay suite — 28 pass (1 unrelated documented tie-break flake, BUG-2270/2279)
• Codex CLI review — CLEAN

The effect_update_depth_exceeded runaway only manifests under the real browser scheduler (not jsdom/vitest), so the E2E overlay tests own the loop regression; the unit tests lock the ref-count semantics.

Fixes BUG-2284. Unblocks #1008 (IDEA-2135), whose E2E inherited this red.

https://claude.ai/code/session_01EZ6yr6pAUFb1uffan912ra