Migrate from Mantine to Tailwind + shadcn/ui

Summary

Migrate policyengine-app-v2 from Mantine 8 to Tailwind CSS v4 + shadcn/ui. Mantine works but creates upgrade pain (v6→v7→v8 were breaking migrations) and locks styling to one library's patterns. Tailwind + shadcn/ui gives us owned components (no library upgrades), Tailwind alignment with policyengine-slides, and the largest React component ecosystem.

Status: complete

All phases implemented in PR #730. Zero @mantine/* imports remain. All Mantine packages removed from dependencies. CI passes (2999 tests, typecheck, lint, build).

Scope

• 287 files changed across the migration
• 192 files previously imported Mantine — all migrated
• 3 Mantine packages removed: @mantine/core, @mantine/dates, @mantine/hooks
• 2 Mantine dev dependencies removed: eslint-config-mantine, postcss-preset-mantine

Phases (all complete)

• Phase 0: Foundation — Tailwind CSS v4 + shadcn/ui installed
• Phase 1: Wrapper components + hooks — Text, Stack, Group, Title, Container, Spinner + useMediaQuery, useViewportSize, useDisclosure
• Phase 2: Static/presentational pages — shared/static/, home/, Footer
• Phase 3: Blog, research, columns — Blog components, column renderers, brand pages
• Phase 4: Report output pages — pages/report-output/ + components/report/
• Phase 5: Form controls — Mantine form components → shadcn equivalents
• Phase 6: Charts + visualization — ChartContainer, maps, TwoColumnView
• Phase 7: AppShell, layout, header, sidebar — MantineProvider + AppShell replaced with Tailwind flex layout
• Phase 8: Pathways — Report/simulation/policy pathway wrappers and views
• Phase 9: Cleanup — All Mantine dependencies removed, theme files deleted

Supersedes #728 (vanilla-extract approach).

The issue states that the migration is complete in PR #730, covering 287 files and removing all Mantine imports and dependencies. Start by reviewing PR #730 and the listed migration phases; the stated completion criteria are passing tests, typecheck, lint, and build with no remaining Mantine packages.