feat(editor): activate git-backed live editing in ProjectEditor (wire dormant ProjectRepoAdapter)

[pparage]

Problem

The full git-backed editing stack is built and unit-tested but not wired into the editor. ProjectRepoAdapter (src/services/projectRepo/adapter.ts) + useProjectRepo (src/composables/useProjectRepo.ts) implement draft-branch autosave, .lock + SharedWorker heartbeat, orphaned-draft recovery, and promote-to-main with PR fallback — all dormant.

ProjectEditor.vue imports only useProjectGitSync (one-shot manual save). Its debounced autosave writes only to localStorage (src/views/ProjectEditor.vue:459-512; see the "if/when backed by a ProjectRepoAdapter" comments at ~461 and ~925). acquireLock / heartbeat / checkLockOwnership have no live callers (only the interface + composable reference them).

Work (sub-tasks)

• Instantiate useProjectRepo; acquire lock on mount / release on unmount (adapter.ts:211-234) — M
• Route the debounced autosave through adapter.autosave() (build ProjectState via buildProjectState() in src/overlay/projectState.ts) — M
• Orphaned-draft modal for multi-tab collision — consume onOrphanedDraft / status === 'lock-lost' (adapter.ts:245-264) — L
• Lock-status badge in the editor header — S
• PR-URL fallback modal on save conflict (today it is only a toast — ProjectEditor.vue:515-547) — overlaps feat(deploy): dirty-state gate + save-and-deploy with PR-URL fallback modal #63 — M
• Dirty-state gate (isDirty) — overlaps feat(deploy): dirty-state gate + save-and-deploy with PR-URL fallback modal #63 — S
• overlay.json editing: load state.overlay from the adapter into the existing VariablesTab (currently reads empty project.overlay) — S

Dependencies

• Requires a git binding (feat(projects): bind a project to a git source/repo (GitBindingModal + project.git) #94) and backend registration (feat(projects): register UI projects with the backend (POST /v1/projects) so they are deployable #95).
• The lock heartbeat posts to POST /v1/projects/{id}/heartbeat, which does not exist server-side — see range42-backend-api#106 (otherwise the worker 404s every 60s once wired).

Relations

• Supersedes the "git-backed projects" portion of the feat(ui): attachments pipeline, serialization, git-backed projects, v1 VM lifecycle #89 epic.
• PR-URL fallback + dirty-state gate overlap feat(deploy): dirty-state gate + save-and-deploy with PR-URL fallback modal #63 — close/fold feat(deploy): dirty-state gate + save-and-deploy with PR-URL fallback modal #63 into those sub-tasks when picked up.