[TD-03] Remove technician keypair from localStorage (security)

[licette32]

Problem

app/src/utils/technicianKeypair.ts generates and stores the technician's secret key in localStorage as JSON. The private key remains exposed in the browser.

This was a shortcut to be able to run the demo with a single wallet. It is unacceptable for production.

Plan

1. Remove technicianKeypair.ts and all its references
2. The complete_maintenance workflow already requires dual signatures — the change is only in the frontend
3. The technician must connect their own Phantom wallet before being able to accept and complete jobs
4. Update the demo instructions in README (already updated, see docs/ARCHITECTURE.md)

Affected files

• app/src/utils/technicianKeypair.ts — remove
• Any component that imports loadOrCreateTechnicianKeypair()

Milestone: v1.1 — Before Demo Day