fix: show the loading placeholder while a doc's tree is fetched

[iftakharul-islam]

Follow-up to #340 (already merged). Fixes a UX regression that came in with the lazy loading.

Tracking issue: weDevsOfficial/wedocs-pro#366

Symptom

Opening a documentation showed a blank screen first, then everything appeared at once with no transition — no skeleton, no animation.

Why

Three things, all introduced or exposed by the lazy loading in #340.

1. The placeholder was gated on the wrong flag. DocListing used the store's loading, which tracks the dashboard's request. During a doc's own child fetch that flag is false, so ! loading was true and the component rendered its "No section or article found" branch for the entire request.

2. The selectors were not subscribed. getSectionsDocs and getDocArticles were called with an empty dependency list:

}, [] );

so they kept returning the value captured on first render and only refreshed when something unrelated forced a re-render. That is what made content arrive late rather than as soon as the fetch resolved.

3. A doc with no sections would wait forever. loadDocTree only called markChildrenLoaded when sections.length was truthy — a guard I added in #340 to make failed requests retry. But that conflates "request failed" with "this doc genuinely has no sections", so an empty documentation never became childrenLoaded.

Changes

• Derive this screen's readiness from its own state: docsLoading || loadingChildren || ! childrenLoaded
• Give the section/article selectors real dependencies ([ id, docs ]) so they track the store
• Mark a branch loaded whenever the request succeeds, empty or not, in a try/finally so loadingChildren always clears; a genuine failure stays unmarked and retries
• Track the requested ID in the effect, so an unmarked branch after a failure cannot become a request loop

Verification

Loading gate checked across every transition:

State	loading	Renders
Mount, nothing started	true	Skeleton
Child fetch in flight	true	Skeleton
Loaded, has sections	false	Content
Loaded, no sections	false	Empty state (not a spinner)
Dashboard still loading	true	Skeleton
Revisit cached branch	false	Content immediately

Confirmed against the live API that the empty case returns HTTP 200 with 0 sections, so it now marks loaded and shows its empty state rather than waiting forever.

Build and no-undef lint clean.

Not yet visually confirmed in a browser — the Playwright profile was locked by a running Chrome instance while I was working, so the skeleton itself has not been seen on screen. The state logic is verified as above, but a quick manual check of the skeleton appearing on a slow doc would be worth doing before merge.