feat(adr-036): migrate App.vue to v2 kind-tagged registry prop

[rubenvdlinde]

Summary

• Replaces the deprecated `customComponents` prop on `CnAppRoot` and `CnObjectSidebar` with the v2 `registry` prop (ADR-036, nextcloud-vue#458, OR #1988).
• Consolidates `src/customComponents.js` (tab components) into `src/registry.js`, removing the split between the two files. All entries are wrapped as `{ kind: 'page', component }` via the `page()` helper function.
• Updates `src/main.js` to import `registry` and pass `registryProp` under the `registry` key.
• Updates `src/App.vue` template bindings (`:registry="registry"` on both `CnAppRoot` and `CnObjectSidebar`) and the props block (`registry` replaces `customComponents`), with JSDoc updated to reference ADR-036.
• Version bumped to `0.1.5`.

Test plan

• Build passes (`npm run build`) — confirmed clean (bundle-size warnings are pre-existing)
• Load decidesk in dev environment; verify the main app shell renders all pages including LiveMeeting and detail tabs
• Confirm no Vue console warnings about unknown `customComponents` prop on `CnAppRoot`
• Confirm sidebar tabs (GovernanceBodyMembersTab, MeetingAgendaTab, etc.) still render in detail views

References

• ADR-036 (kind-tagged registry prop)
• nextcloud-vue#458
• OR #1988