Fix snapshot calendar and instance tab colors across themes

Problem

The snapshot calendar uses opaque light-theme fills that do not compose correctly on dark surfaces. Snapshot-count badges force a white background, and instance tabs inherit incorrect link colors in both themes.

Expected

Use theme-safe translucent calendar fills and Material UI theme tokens for tab text, dividers, and disabled states.

Validation

• Shared package build
• CE build and test suite
• Focused Cypress render on a dark surface

Implementation: !1187 (merged)