feat(playground): canonical /playgrounds/:id URL for One-linked playgrounds

[johnleider]

Summary

Implements canonical URL routing for Vuetify One playgrounds, making /playgrounds/:id the canonical URL for One-linked playgrounds (parity with play.vuetifyjs.com).

Changes

New Route: /playgrounds/:id

• Added pages/playgrounds/[id].vue for the canonical playground route using vue-router/auto-routes file-based routing
• Route-provided ID is injected via providePlaygroundRoute for consumption by usePlaygroundFiles

URL Navigation

• Updated useOnePlaygrounds to use vue-router for URL synchronization instead of history.replaceState
• Open dialog uses router.push for explicit navigation (creates history entry)
• Save operations use router.replace for seamless URL updates

Backwards Compatibility

• Legacy /?playground=<id> URLs redirect to /playgrounds/<id> with hash preserved
• Local/untitled playgrounds remain on / with hash-only share links unchanged

Documentation

• Added brief note about canonical URLs and sharing in intro.md

Acceptance Criteria

• Route /playgrounds/:id loads that One playground
• Open/Save navigations use router.push/replace to /playgrounds/:id, preserving hash
• Local/untitled playgrounds stay on / (existing hash-only share links unchanged)
• Reverse compat redirect: /?playground=<id> → /playgrounds/<id> (preserve hash)
• Brief docs note on canonical URL

Fixes #838