feat(demo-admin-dashboard): add reusable demo message template library

[Maximum-Prosper]

Closes #207

Changes

• Added a reusable demo message template library inside src/features/demo-admin-dashboard/.
• Introduced template scenario metadata to group templates into reviewer-friendly flows.
• Added an in-memory template registry with deterministic lookup and duplicate-id protection.
• Added deterministic template fixtures derived from templates and scenario metadata.
• Exported the new library pieces from the feature root for local consumers.
• Added local documentation for the template library and linked it from existing dashboard docs.
• Added tests for template scenario mapping, registry behavior, and fixture determinism.

Testing

• git diff --check
• Added/updated Vitest coverage in:
  • src/features/demo-admin-dashboard/__tests__/messageTemplates.test.ts
  • src/features/demo-admin-dashboard/__tests__/templateRegistry.test.ts
  • src/features/demo-admin-dashboard/__tests__/templateFixtures.test.ts

Notes

• All changes stay inside src/features/demo-admin-dashboard/.
• No main app, routing, or production inbox wiring was added.

[Maximum-Prosper]

@kryputh Please kindly review and merge

[Maximum-Prosper]

@kryputh Please kindly review and merge.