UI: implement the accessible application shell, theme, and settings

Parent: #5451

Summary

Build the shared Mantine application shell, responsive navigation, theme, persistent settings, and accessibility foundations used by every v3alpha-backed page.

Application shell

• Centralize page and route metadata so the Router and Header do not maintain duplicate definitions.
• Build a responsive Mantine AppShell with desktop navigation and a keyboard-accessible mobile burger/drawer.
• Include current-route state, Alertmanager branding and favicon, Documentation, and a route-prefix-safe Classic UI link.
• Add a real not-found page and reusable loading, empty, unavailable, and error states.
• Reset page error boundaries on route changes and lazy-load substantive routes where useful.
• Hide routes whose v3alpha services have not landed rather than exposing non-functional placeholder pages.

Theme and settings

• Define Alertmanager theme tokens, spacing, radius, typography, contrast, and focus behavior centrally.
• Support automatic, light, and dark color schemes and persist explicit selection.
• Integrate/rebase #5106 where suitable and include its requested interaction and accessibility tests.
• Add persistent settings for first day of week, default silence creator, expand-all alert groups, local/UTC time display, and page sizes.
• Preserve compatible Elm localStorage keys (firstDayOfWeek, defaultCreator, and groupExpandAll) and tolerate malformed or missing values.

Accessibility and responsiveness

• Restore browser zoom; do not use user-scalable=no.
• Use semantic page headings, landmarks, table structures, labels, and status/error announcements.
• Ensure all navigation and interactive controls work with keyboard and visible focus.
• Correct the current mobile navigation gap and title visibility breakpoints.
• Prevent fixed-width skeletons, tables, code blocks, labels, and menus from overflowing narrow viewports.
• Do not globally suppress keyboard-equivalent click lint rules.

Acceptance criteria

• One route registry drives navigation and route rendering.
• Every available page is usable by keyboard at mobile and desktop widths.
• Mobile navigation exposes all routes and actions available on desktop.
• Auto/light/dark theme selection is clear, accessible, and persistent.
• Existing Elm preferences migrate without throwing and malformed storage values fall back safely.
• Documentation and Classic UI links work at root and non-root route prefixes.
• Not-found, loading, empty, unavailable, and error states are consistent and announced appropriately.
• Browser zoom, semantic headings/tables, focus visibility, labels, and responsive overflow pass automated and manual checks.
• Interaction tests cover navigation, theme cycling, settings persistence, error resets, and mobile controls.

Dependencies and related work

• Runtime UI path/bootstrap: #5486.
• Can proceed in parallel with Connect client work in #5487.
• Theme work should coordinate with #5106 and dark-mode request #2556.
• Local/UTC settings relate to #3861.

Likely code areas

• ui/mantine-ui/src/{App,main,theme}.tsx
• ui/mantine-ui/src/components/Header*, ErrorBoundary.tsx, and new shell components
• new route and settings modules/tests
• ui/mantine-ui/index.html, biome.json
• shared test utilities