[OPE-166] Post-mortem template selector in generation UI

[singret]

Summary

• Auto-selects first template on load so every user lands on a named template rather than the silent "Default" fallback
• Adds TemplatePills component to the empty state — visible at 1080p without scrolling, above the action buttons
• Generate button label reflects selected template (e.g. "Generate · Infrastructure")
• Single-template case: one muted non-interactive pill still communicates the concept
• Zero-template case: no pills, button reads "Generate with AI" — identical to before
• Existing PM case: AI ▾ dropdown template selector now inherits the pre-selected value

Changes

• frontend/src/components/incidents/PostMortemPanel.tsx — single file, no backend or API changes

Closes OPE-166