fix(reader): make annotation tooltips theme-aware

[cha1latte]

Summary

• derive the annotation tooltip surface, border, and Markdown colors from the active reader theme
• keep tooltip text readable in light and sepia themes instead of forcing a dark surface
• cover the theme contract with a focused regression test

Verification

• Expo tests: 15 passed
• Expo TypeScript check: passed
• new helper and regression test pass Biome