test(e2e): read sidebar categories in one snapshot inside polls (#1728)

The "allows restoring live categories after every category is hidden"
Electron test failed intermittently on shard 1 across unrelated PRs and
master. The app hid every category correctly (the failure screenshot
shows "No categories available"), but readVisibleSidebarCategoryNames
looped over count() with per-row nth(index) reads. When the sidebar
removed a row between isVisible() and textContent(), textContent()
auto-waited for the missing element, so the expect.poll predicate never
returned and the poll timed out with "waiting on the predicate" instead
of retrying.

Read the visible rows with a single evaluateAll() snapshot in a shared
sidebar-categories.e2e-support.ts helper, use it for the category
picker in category-management and backup-roundtrip too, and record the
rule in the validation map.

Co-authored-by: 4gray <fourgray@proton.me>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture/validation-map.md

@@ -181,6 +181,13 @@ and JSON summary output. CI uploads the merged Tier A report to Codecov with the
 Use atomized E2E targets when available, for example
 `pnpm nx run web-e2e:e2e-ci--src/xtream.e2e.ts`.
 
+Inside `expect.poll`, read a changing list in one DOM snapshot
+(`allTextContents()` or `evaluateAll()`, as in
+`apps/electron-backend-e2e/src/sidebar-categories.e2e-support.ts`), not by
+looping over `count()` with per-row `nth(index)` reads. Those reads auto-wait,
+so a row removed mid-loop hangs the predicate until the poll times out instead
+of letting it retry.
+
 Playwright coverage is measured semantically by tags and critical journeys, not
 by a source-line percentage. E2E reports should use tags such as `@critical`,
 `@electron`, `@web`, `@xtream`, `@stalker`, `@m3u`, `@search`, `@epg`,