Insights panel: unify section borders into card style

[erikdarlingdata]

Summary

• Replace tinted per-section backgrounds (brown/green/blue/purple) in the Plan Insights panel with a single InsightCard style (lifted #1E2228 surface, #353942 border, 8px radius, subtle box-shadow).
• Section identity now lives only in the accent-colored header text; headers share a new InsightHeader class (14pt SemiBold).
• Bumps the panel MaxHeight 220 → 240 and adds inner-grid margin so the card shadows have room to render.

Addresses the UI/UX feedback in .internal/ui_ux_feedback.md (Phase 1–3 of the technical plan).

Test plan

• Launch app, open a .sqlplan with runtime stats — verify all 5 cards render with the new lifted surface and shadows.
• Confirm wait-stats proportional bars retain their per-category colors and read better against #1E2228.
• Confirm Missing Index SQL still wraps inside its card.
• Confirm Server Context card hides cleanly when there's no metadata.
• Verify Runtime Summary perf colors (red/orange/white) still pop.

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