fix(web): forward csp nonce to theme script

[lyzno1]

Fixes #32185

Summary

• pass the CSP nonce from root layout into next-themes ThemeProvider
• set forwarded request headers before creating NextResponse.next() so RSC can read x-nonce

Testing

• pnpm --dir web exec eslint app/layout.tsx proxy.ts
• pre-commit eslint on staged files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.80%. Comparing base (c5ac191) to head (96a1b16).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #35960      +/-   ##
==========================================
- Coverage   85.80%   85.80%   -0.01%     
==========================================
  Files        4463     4463              
  Lines      209559   209559              
  Branches    39250    39250              
==========================================
- Hits       179814   179808       -6     
- Misses      26552    26558       +6     
  Partials     3193     3193              

Flag	Coverage Δ
dify-ui	94.23% <ø> (ø)
web	86.75% <ø> (-0.01%)	⬇️

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