feat(react): grouped list and reminder cards

[kauereinbold]

Summary

• Replace the MUI table on the list page with the redesigned grouped list: sections Overdue/Today/Upcoming/Done (empty groups hidden, items sorted ascending by limitDate), serif section headers with counts and rules
• New ReminderCard component per design handoff: 24px circle checkbox with optimistic toggle (rollback on API error), title/description, date pill (accent tint when overdue; Today/Tomorrow/Yesterday/'N days late'/short date labels), 34px edit icon button
• Pure grouping/date logic in util/reminderGroups.ts, built on the feat(react): design tokens and fonts foundation #326 design tokens

Fixes surfaced during E2E and manual testing

• api: updateReminder initialized errors as [] (truthy), making the optimistic rollback fire on every successful update
• api: getErrors crashed (Object.keys on undefined) on the flat { message } 400 body returned by the Go/C++ APIs; now mapped to BadRequest with fallbacks
• ReminderForm never rendered BadRequest errors; AlertError now shows InternalServer or BadRequest
• Page wrapper had fixed 6rem padding, collapsing cards on small viewports; now uses the design gutter token

Cypress

• reminders-list, reminders-integration, and commands updated from table selectors to the card UI (kept the post-deploy Pages run green)
• New create-page test covering the flat { message } 400 shape
• New visual-check spec (@visual) capturing per-viewport screenshots for design review

Out of scope, tracked in siblings: header/sidebar/week progress (#328), search/filters (#329), modal + delete confirm (#330), mobile shell (#331), full MUI removal (#332).

Closes #327

Test plan

• jest: 87/87 passing
• cypress: 40/40 across 6 specs against local dev + docker api profile
• next lint and tsc --noEmit clean
• Manual pass on http://localhost:3000 with seeded data; screenshots reviewed at 1280/768/375