feat(standalone): env-selected Yjs provider abstraction (ysync 0004)

Generalize the collab transport into a provider registry (none |
broadcastchannel | partykit | hocuspocus), one active per env. startCollab
takes { provider, room } and awaits provider.whenSynced() before seed/adopt.
Network libs lazily imported; room id from @pcbjam/shared collabRoomId.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/standalone/package.json

@@ -11,6 +11,7 @@
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
+    "@hocuspocus/provider": "^4.1.1",
     "@pcbjam/shared": "workspace:*",
     "@radix-ui/react-dialog": "^1.1.4",
     "@radix-ui/react-label": "^2.1.1",
@@ -25,6 +26,7 @@
     "react-router-dom": "^6.28.1",
     "tailwind-merge": "^2.6.0",
     "tailwindcss-animate": "^1.0.7",
+    "y-partyserver": "^2.2.0",
     "yjs": "^13.6.18",
     "zod": "^3.24.1"
   },