Upgrade Vite to 8 and @vitejs/plugin-react to 6

[gabitoesmiapodo]

User story / Problem statement

Currently Vite is two majors behind (6 vs 8) and @vitejs/plugin-react is two majors behind (4 vs 6) in carpincho-wallet and dapp/frontend.

Expected outcome

Both packages build and dev-serve on Vite 8 with plugin-react 6.

Acceptance criteria

• Confirm @tailwindcss/vite supports Vite 8 before bumping
• vite 6.4.3 → 8.x (carpincho-wallet, dapp/frontend)
• @vitejs/plugin-react 4.7.0 → 6.x (carpincho-wallet, dapp/frontend)
• vite build and vite dev work in both packages
• Lint + typecheck + tests pass

Technical notes

Part of #76. Couple the Vite and plugin-react majors. Verify the Tailwind Vite plugin first since it is the most likely incompatibility.