Wire PageDesigner's manifest load/save (it renders with an empty manifest)

[rubenvdlinde]

Follow-up from #23 (re-applying openbuilt-page-editor onto the Tier-4 shell). src/views/PageDesigner.vue is now reachable at /builder/:slug/pages and the three-pane visual editor (Pages / Menu builders + per-page-type sub-editors + live validation) renders — but it starts from the default empty manifest: there's no plumbing to load the virtual app's current manifest from GET /api/applications/{slug}/manifest (or the Application object) and to persist edits back.

In the original PR #4 that plumbing lived in the competing replacement ApplicationEditor.vue "Design" tab + the applicationEditor Pinia store (src/store/modules/applicationEditor.js, which expects the lib's createObjectStore semantics — fetchObject / saveObject / getError — that development's src/store/modules/object.js doesn't fully provide). Re-introducing that shell was out of scope for #23 (the merged shell has the list + Editor/History/Diff tabs). applicationEditor.js is currently an inert orphan (nothing imports it; Pinia stores are lazy, so it's harmless).

To finish:

• Either migrate src/store/modules/object.js → the lib's createObjectStore (so applicationEditor.js works), or rework applicationEditor.js to use the existing object.js API.
• Wire PageDesigner.vue to load applicationUuid's manifest on created() (via the slug from $route.params.slug → getManifest or the Application object) and to PUT/PATCH the edited manifest back (a PATCH /api/applications/{slug} or going through OR's objects API).
• Add a write endpoint if one doesn't exist (the read side, applications#getManifest, is already there).
• Decide the entry point: a "Design" link in the virtual-app context (BuilderHost secondary nav) and/or a button in the ApplicationEditor detail that opens /builder/:slug/pages.

Related: #4 (the spec), #19 (publish-path transition gap).

[rubenvdlinde]

Done — PR #27 (merged to development, 35f4046). Added PageDesignerHost.vue: resolves the :slug route param to its Application via OR's objects API, seeds the editor with the stored manifest, persists edits back with a PUT, "Save pages" button + back/builder links + a not-found state. customComponents.PageDesignerView now points at the host; ApplicationEditor's detail got a "Design pages" router-link; the inert orphan src/store/modules/applicationEditor.js was deleted. Verified in the dev container: /builder/hello-world/pages loads the hello-world manifest with the Pages/Menu builders populated and the validator side-panel running against the real manifest.