deps: update deps across the repo to latest

[devdumpling]

Summary

• Upgrade all dependencies to latest versions (React 19, Tailwind 4, Vite 7)
• Migrate to Tailwind 4's new Vite plugin architecture
• Centralize dependency management with pnpm catalogs

Changes

Dependency Upgrades

Package	Before	After
react	18.x	19.x
tailwindcss	3.x	4.x
vite	6.x	7.x
svelte	5.45.2	5.45.3
vitest	4.0.0	4.0.14
typescript	5.7.x	5.9.x

Tailwind 4 Migration

• Switched from PostCSS plugin to @tailwindcss/vite
• Migrated theme config from tailwind.config.js to CSS @theme directive
• Removed postcss.config.js and tailwind.config.js

pnpm Catalogs

• All non-bespoke dependencies now managed in pnpm-workspace.yaml
• Single source of truth for versions across all 6 workspace packages
• Only exception: @rocicorp/zero (rapidly iterating, kept inline)