feat(react): desktop shell: header, sidebar nav, week progress

[kauereinbold]

Summary

• New AppHeader component: sticky header with serif page title, today's date, search pill (magnifier icon, no focus ring), and the New reminder primary button (replaces the old Create Reminder button).
• New Sidebar component: sticky aside with All/Today/Upcoming/Done nav (counts, active state with accent dot and aria-current) and the THIS WEEK progress block (uppercase label, 40px serif percentage, 5px bar, overdue-aware caption).
• reminderGroups util grows viewCounts and weekProgress (percent done of reminders due within 7 days, overdue included, matching the prototype).
• List page becomes the two-column shell (max-width 1280px, clamp gutters); reminder list renders inside the main column. Three nav color tokens added to globals.css; unused page.module.css deleted.
• Scope split per the redesign plan: search input and view selection hold state only; list filtering ships with feat(react): search and view filters #329, create/edit modal with feat(react): create/edit modal and delete confirmation #330, mobile layout with feat(react): mobile layout under 760px #331, empty states with chore(react): remove MUI styling, empty states, test updates #332.

Closes #328

Test plan

• npm test: 100/100 passing, including new AppHeader, Sidebar, util, and list shell tests.
• next lint and next build clean.
• Cypress against local prod build: all 6 UI specs green, including a new desktop shell spec (header, nav counts, active-state switching, week progress). Button rename applied across specs and commands. api-dotnet.cy.js needs the live .NET API and is unaffected by this change.
• Manual smoke test against the docker API stack (nginx :9999): sticky header/sidebar, counts, progress math, hover states, optimistic toggle updating the progress block.