React Review Audit

Fix the following React Review diagnostics in my codebase.

## Warnings (20)

1. [warning] design-no-default-tailwind-palette — src/components/NotFound.tsx:6
   text-gray-600 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

2. [warning] design-no-default-tailwind-palette — src/components/NotFound.tsx:6
   text-gray-400 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

3. [warning] design-no-bold-heading — src/routes/_authed/posts.$postId.tsx:24
   font-bold on <h4> crushes counter shapes at display sizes — use font-semibold (600) or font-medium (500)

4. [warning] html-has-lang — src/routes/__root.tsx:100
   Missing lang attribute.

5. [warning] tanstack-start-server-fn-validate-input — src/routes/__root.tsx:18
   Server function handler accesses data without inputValidator() — validate inputs crossing the network boundary

6. [warning] design-no-default-tailwind-palette — src/components/DefaultCatchBoundary.tsx:28
   bg-gray-600 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

7. [warning] design-no-default-tailwind-palette — src/components/DefaultCatchBoundary.tsx:28
   bg-gray-700 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

8. [warning] design-no-default-tailwind-palette — src/components/DefaultCatchBoundary.tsx:35
   bg-gray-600 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

9. [warning] design-no-default-tailwind-palette — src/components/DefaultCatchBoundary.tsx:35
   bg-gray-700 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

10. [warning] design-no-default-tailwind-palette — src/components/DefaultCatchBoundary.tsx:42
   bg-gray-600 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

11. [warning] design-no-default-tailwind-palette — src/components/DefaultCatchBoundary.tsx:42
   bg-gray-700 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

12. [warning] no-pure-black-background — src/components/Auth.tsx:13
   Pure black background (bg-black) looks harsh — use a near-black tinted toward your brand hue (e.g. bg-gray-950)

13. [warning] design-no-default-tailwind-palette — src/components/Auth.tsx:14
   bg-gray-900 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

14. [warning] design-no-bold-heading — src/components/Auth.tsx:15
   font-bold on <h1> crushes counter shapes at display sizes — use font-semibold (600) or font-medium (500)

15. [warning] no-prevent-default — src/components/Auth.tsx:16
   preventDefault() on <form> onSubmit — form won't work without JavaScript. Consider using a server action for progressive enhancement

16. [warning] design-no-default-tailwind-palette — src/components/Auth.tsx:31
   border-gray-500 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

17. [warning] design-no-default-tailwind-palette — src/components/Auth.tsx:31
   bg-gray-800 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

18. [warning] design-no-default-tailwind-palette — src/components/Auth.tsx:42
   border-gray-500 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

19. [warning] design-no-default-tailwind-palette — src/components/Auth.tsx:42
   bg-gray-800 reads as the Tailwind template default — use zinc (true neutral), neutral (warmer), or stone (warmest)

20. [warning] tanstack-start-server-fn-validate-input — src/utils/posts.ts:29
   Server function handler accesses data without inputValidator() — validate inputs crossing the network boundary