Readable text on custom card backgrounds (#77)

[SlyDave]

Summary

• Apply contrastTextColour to task card title and description when a custom background is set.
• Leave theme text colours when background is none; contrast follows card background luminance, not theme alone.

Closes #77

Acceptance criteria

• Custom background → title and body use light/dark contrast from luminance — taskCardContrastText + inline colour on title and description.
• Background none → normal theme colours — helper returns null; text-highlighted / text-muted remain.
• Light and dark theme with custom background → contrast from card background — colour is computed from the hex background only.

Test plan

• Card with light background shows dark title/body
• Card with dark background shows light title/body
• Card with no background uses theme text colours in light and dark mode

[SlyDave]

QA verdict: approved

Verified against #77 acceptance criteria on frontend/issue-77 (local Nuxt at :3077) plus unit tests (taskCardContrast.test.ts 3/3). Seeded todo.tasks.v1 with light (#ffffff), dark (#000000), and none backgrounds.

Criterion	Result	Evidence
Custom bg → luminance contrast on title/body	Pass	White card → #111827 / rgb(17,24,39); black card → #f9fafb / rgb(249,250,251)
Background none → theme colours	Pass	No inline colour; theme oklch(...) classes remain
Custom bg contrast ignores theme alone	Pass	Under dark theme, white card still uses dark contrast text

CI was in progress at review start; unit + browser checks cover the ACs.