chore: set NEXT_PUBLIC_ENABLE_FEATURE_PREVIEW default to true

[Copilot]

Summary

Sets NEXT_PUBLIC_ENABLE_FEATURE_PREVIEW default to true across all configuration sources:

• web/env.ts — Zod schema default
• web/docker/entrypoint.sh — shell fallback
• docker/.env.example, docker/envs/core-services/web.env.example, web/.env.example — example configs

From Copilot.

Screenshots

Before	After
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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.28%. Comparing base (f4ec608) to head (f6f00e6).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #38362      +/-   ##
==========================================
- Coverage   85.29%   85.28%   -0.01%     
==========================================
  Files        4989     4989              
  Lines      264000   264036      +36     
  Branches    50097    50115      +18     
==========================================
+ Hits       225166   225192      +26     
- Misses      34419    34429      +10     
  Partials     4415     4415              

Flag	Coverage Δ
api	85.51% <ø> (+<0.01%)	⬆️
dify-ui	94.93% <ø> (ø)
web	84.86% <ø> (-0.01%)	⬇️

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