[Demo only DO NOT MERGE] feat(feedback): prototype feedback and in-product notifications

[CORCTON]

Related: #1789

Goal

Validate a lightweight feedback flow in the existing XBuilder experience: users submit structured feedback, administrators review it, and replies reach users through persistent in-product notifications.

Demo scope

• Add a global feedback entry to Profile.
• Submit a title, description, and optional attachments.
• Review feedback in an administrator-facing feedback pool.
• Reply to feedback or mark it as requiring no reply.
• Deliver administrator replies through a reusable in-product notification entry with an unread indicator and detail view.

Demo shortcuts

• Feedback and notification data is stored in localStorage.
• Attachments use local object URLs and are not uploaded.
• The Admin feedback route bypasses production authorization so the demo can be evaluated directly.
• No backend APIs, persistence, delivery service, or production permission model are included.
• This PR is for interaction and feasibility validation only and must not be merged.

Verification

• pnpm lint
• pnpm format-check
• pnpm type-check
• pnpm exec vitest run src/components/feedback-demo/model.test.ts
• git diff --check