#21431 fix(cmux-next): make Apps failures recoverable

Summary

• add explicit catalog and detail loading/error/not-found states with retry and back actions
• preserve toolbar and selection while recovering a failed Apps request
• isolate the Apps gallery browser lane and cover recovery in Chromium and WebKit

Verification

• bun run typecheck
• bun run lint
• bun run check:browser-lanes
• bun test test/gallery-coverage.test.ts
• CMUX_BROWSER_TESTS=1 timeout 300 bun test --timeout 120000 test/apps-flow.test.ts -t "retries a catalog failure|shows a not-found detail|retries a detail error" — 6 pass, 18 expects; Chromium and WebKit
• git diff --check

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Summary by cubic

Makes Apps catalog and detail failures recoverable with explicit loading, error, and not-found states and retry/back actions, without dropping the toolbar or the selected app.

• Catalog errors show a Retry banner with stale rows staying visible during retry; detail failures keep the route and selection, and Retry reloads only the selected app.
• Loading states no longer reuse failure text: the grid and detail say "Loading…", and a missing app says "App not found" instead of the search empty state.
• A deep-linked detail settles after a catalog failure, or sits idle with no client, instead of loading forever; the banner's Reload retries both, and a failed empty catalog no longer claims "No apps match".
• Browser tests cover catalog retry, not-found, and detail error recovery in Chromium and WebKit; stage frames can opt into a single entry path so tests don't cold-load the entire registry.