Polish Incident UX and add redacted summary export

[XploitGost]

Summary

• improve Incident readability, severity badges, empty state, selection behavior, and mobile spacing
• add protected read-only Markdown summary generation and copyable UI preview
• keep export bounded, centrally redacted, in-memory, and non-persistent
• document incident reading and summary export workflow

Validation

• Backend: 341 tests passed
• Frontend: 52 tests passed
• Frontend build: passed
• pip check: passed
• npm production audit: 0 vulnerabilities
• Electron smoke: passed
• Desktop and 375px mobile visual QA: no horizontal overflow

Boundaries

No correlation-rule changes, AI, server mode, response actions, blocking, command/control, remote shell, TLS decryption, MITM, credential collection, browser data inspection, or Agent raw forwarding.