fix(imagegen): show diffusers version info and fix upgrade flow

[apapi]

Problem

Closes #196

The Diffuser upgrade feature had several issues:

1. No version info in UI — The upgrade UI did not show the current installed version or the latest available version, so users could not tell whether an upgrade was needed.
2. Button state incorrect after upgrade — After a successful upgrade, the button stayed enabled and version info was stale.
3. Upgrade did not actually work — Version constraints in package specs (e.g. diffusers>=0.34.0) caused --upgrade to consider the installed version as already satisfying the constraint, so no newer version was pulled.
4. State lost on page refresh — Navigating away and back did not re-query the actual installed version, so the UI showed stale info.

Fix

Backend (internal/imagegen/runtime.go)

• New StatusWithVersions() method enriches Status() with:
  • installedPackageVersion() — probes venv Python via importlib.metadata.version()
  • latestPyPIPackageVersion() — parses the configured mirror PEP 503 simple index page, falls back to pypi.org JSON API
• New stripVersionSpecs() — removes version constraints before --upgrade so the latest version is installed
• InstallWithProgressOptions returns StatusWithVersions after upgrade so the frontend gets updated version info immediately

Backend handler (internal/server/handlers_images.go)

• GET /api/image-runtime uses StatusWithVersions

Frontend (web/src/pages/Settings.tsx, web/src/api/client.ts)

• Fetch runtime status on page mount to get current and latest versions
• After upgrade, re-fetch actual installed version and compare to determine success
• Button states: disabled while upgrading / "Already upgraded" when versions match or just upgraded / "Upgrade Diffuser" when newer version available
• Version display: current version + latest version (latest only shown when different)
• getImageRuntimeStatus uses cache: "no-store" to prevent stale responses

API docs (openapi/local-api.json)

• RuntimeStatus schema: added diffusers_version and diffusers_latest_version fields

i18n (web/src/i18n.ts)

• Added zh/en keys: current version, latest version, already upgraded, already at latest

Files changed

File	Change
internal/imagegen/runtime.go	Version query, constraint stripping, StatusWithVersions
internal/server/handlers_images.go	GET handler uses StatusWithVersions
openapi/local-api.json	Schema: two new fields
internal/server/static/openapi/local-api.json	Synced
web/src/api/client.ts	Type fields + cache no-store
web/src/i18n.ts	zh/en i18n keys
web/src/pages/Settings.tsx	Version display, button states, post-upgrade re-fetch

Verification

• gofmt / go build / go vet / go test ./internal/server/ — all pass
• tsc --noEmit / vite build — pass
• Manual test: downgraded to 0.39.0, UI shows current 0.39.0 / latest 0.40.0, click upgrade, success, button disabled showing "Already upgraded", version updated to 0.40.0, navigate away and back, state correct