perf: unmount idle Marketplace/Admin/Docs/ApiTest views

[aguung]

Summary

• Classic layout (the default) kept every center view mounted permanently, toggling only a CSS hidden class — so a view's own background polling ran forever regardless of which tab was actually visible, and its state/DOM never got released across a long session.
• Converts Marketplace, Admin, Docs, ApiTest to conditional mounting (mount only the active view), matching the pattern FocusShell already uses correctly for layoutMode: "focus".

Problem

• web/src/os/Desktop.tsx's Classic-mode block rendered every center view unconditionally, hiding inactive ones via a hidden class rather than not rendering them.
• Concretely: MarketplaceApp runs three separate setIntervals (including cloud-touching calls) and AdminApp runs a 15s poll — both continuously, even when the user is on the Widgets or Terminal tab and never looks at Marketplace/Admin again for the rest of the session.

Solution

• Replace each of the four views' <div className={... ${view === "x" ? "" : "hidden"}}> wrapper with {view === "x" && (<div ...>...)} — React unmounts the view when its condition goes false, running its effect cleanup and releasing its state/DOM.
• Widgets, CenterTerminal, AiChatApp are untouched and stay always-mounted: Widgets is the default/home view, and Terminal/AI Chat hold live session state that must survive switching away and back.
• No behavior change to the presence/SSE heartbeat, layoutMode: "focus" (already correct), or any mobile-specific layout (none exists in this codebase today).
• Safe by construction: every setInterval in the affected views already has a matching clearInterval in its effect cleanup, so React's unmount lifecycle stops them automatically — this required no changes to MarketplaceApp.tsx/AdminApp.tsx themselves.

Changed files

File	Change
web/src/os/Desktop.tsx	Four views converted from CSS-hidden to conditionally mounted.

No conflict with other open PRs

This only touches web/src/os/Desktop.tsx, which none of #6, #7, #8, or #9 touch — safe to merge in any order relative to them.

Test plan

• tsc -b --noEmit and oxlint pass on the changed file
• Manually traced: every setInterval in MarketplaceApp/AdminApp has a matching clearInterval in the same effect's cleanup, so unmounting reliably stops them; DocsApp/ApiTestApp have no timers to clean up
• Visual confirmation in a running browser (tabs still render correctly; Network tab activity for Marketplace/Admin stops when switching away and resumes when switching back) — not done in this environment (no browser-automation tooling available); please check before merging