feat(projects): bind a project to a git source/repo (GitBindingModal + project.git)

[pparage]

Problem

There is no UI to associate a project with a git repo, so projects are never git-bound. useProjectGitSync.buildPushArgs() returns null when project.git is absent (src/composables/useProjectGitSync.ts:47), so "Save" silently falls back to local-only and nothing ever reaches git. Because binding never happens, neither git sync nor backend project registration can occur downstream.

What already exists

• ProjectGitBinding type is fully defined — src/composables/useProjectGitSync.ts:19-31.
• inventoryStore holds configured git sources + PATs (src/stores/inventoryStore.ts).
• ProjectRepoAdapter supports both shared_repo_subdir and dedicated_repo strategies.
• Project creation entry points: src/views/CatalogList.vue:108, src/views/CatalogEntryDetail.vue:182,196 (currently projectStore.createProject() → localStorage only).

Work

• GitBindingModal.vue: pick source (from inventoryStore), repo owner/name, branch strategy (shared_repo_subdir | dedicated_repo), subdir.
• Persist binding to project.git (extend the project model in projectStore.js).
• "Link to Git" affordance in the editor settings tab.
• Auto-bind when forking from a catalog entry that already has a git source (CatalogEntryDetail.vue).

Dependencies / relations

• Prerequisite for feat(projects): register UI projects with the backend (POST /v1/projects) so they are deployable #95 (project registration — backend needs source_id + repo coords) and feat(editor): activate git-backed live editing in ProjectEditor (wire dormant ProjectRepoAdapter) #96 (git-backed live editing).
• Part of the git-backed-projects portion of the feat(ui): attachments pipeline, serialization, git-backed projects, v1 VM lifecycle #89 epic.

Size: L (~150 lines incl. modal + source picker).