feat: remember default highlight color

[codedogQBY]

Summary

• remember the last used highlight color in reading settings and use it as the default for new highlights
• make the highlight button apply immediately for fresh selections, while existing highlights can still open the color picker and change color
• re-render highlight annotations when color changes and add the missing desktop purple color mapping
• apply the same default-color behavior on mobile

Fixes #279

Verification

• pnpm --filter app build
• pnpm --filter @readany/app-expo exec tsc --noEmit
• pnpm --filter @readany/core test
• git diff --check