Fix version-switcher stuck-on-unversioned bug and Environments modal duplicates

[SayantanCode]

Summary

Three more real bugs found while manually testing the docs UI against a real production API (600+ endpoints):

• Stuck on "Unversioned." The app's default version key (usually "v1", from schema-engine's resolveVersion fallback) only ever appears in the version switcher once a real ApiVersionDoc is explicitly created for it. A brand-new project has real data under it but no such record — so switching to "Unversioned" (always listed) before ever visiting "Manage versions…" was a one-way trip with no way back except manually creating a version with that exact name. VersionSwitcher now always surfaces the default version key as a selectable option when it isn't a real stored version yet.
• Stale tree during version switches. The loading-state fix from the previous PR only covered the very first load — switching versions afterward could leave the previous version's folders/content on screen for however long the new version's fetch took, indistinguishable from the switch silently failing. Now cleared and re-armed on every version change.
• Environments modal allowed duplicate creation. No saving/disabled state on the Save button meant a double-click fired two identical create requests before the first even resolved. Also no duplicate-name check at all, and the form never reset after a successful creation.

Changes

• VersionSwitcher.tsx: extracted isDefaultVersionPhantom (pure, tested) and synthesizes a selectable entry for the app's default version key when needed.
• DocsApp.tsx: renamed initialLoadPending → specLoadPending, now re-armed (and folders/doc cleared) on every version switch, not just first load.
• EnvironmentsModal.tsx: added saving state (disables Save/Delete/Close, shows "Saving…"), resets the form after a successful creation, added a case-insensitive duplicate-name check with an inline error.

Test plan

• pnpm build — passes
• pnpm lint — clean
• pnpm check:boundaries — clean
• pnpm test — 657/657 passing (4 new tests for isDefaultVersionPhantom)
• CI green on this PR