refactor(web): split into GPL standalone editor + thin reference backend

Reduce web/ to a single generic editor app plus a minimal example backend,
moving all project-specific/app code out to the closed root repo:

- apps/frontend -> standalone (@pcbjam/standalone): the GPL editor. Strips the
  project-management UI (ProjectsPage/ProjectDetailPage/UploadDropzone and the
  write half of lib/api.ts). WasmTool now takes fetchBytes + assetBaseUrl as
  injected props (decoupled from the API client) so it can be driven by either
  a backend or a local folder. New HomePage (local-folder loader + backend
  project list) and read-only ProjectView. BroadcastChannel collab unchanged.
- backend (@pcbjam/backend-example): thin Fastify+ts-rest reference impl of the
  @pcbjam/shared contract — serves a single project off the local filesystem
  (PROJECT_DIR), no DB/auth/uploads.
- packages/contract -> web/pcbjam-shared (git submodule, MIT @pcbjam/shared).
- Remove apps/server, packages/storage, packages/contract, docker-compose
  (server + storage move to the closed root repo; preserved via subtree
  branches). Rewrite pnpm-workspace/turbo/.env for the frontend-only layout.
- Add tests/fixtures/demo so the editor + example backend run out of the box.

Standalone typechecks + builds; backend serves the contract end-to-end.

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

.gitmodules

@@ -6,3 +6,7 @@
 	path = wxwidgets
 	url = git@github.com:emergence-engineering/wxWidgets.git
 	branch = wasm-port
+[submodule "web/pcbjam-shared"]
+	path = web/pcbjam-shared
+	url = git@github.com:emergence-engineering/pcbjam-shared.git
+	branch = main