feat(web): unify create_app tracking and persist external attribution

[CodingOnStar]

Summary

This PR refines app creation tracking and consolidates all create-related reporting into a single create_app event.

• persist external attribution in sessionStorage to avoid losing it across registration/login and page switches
• support preserving attribution when entering from Studio and creating later from Explore
• remove consumed utm_source / utm_campaign / slug from the URL after attribution is captured
• clear remembered attribution after successful create_app reporting to avoid duplicate external attribution

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

❌ Patch coverage is 96.62162% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.87%. Comparing base (5bc0f95) to head (a801499).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
web/utils/create-app-tracking.ts	96.58%	4 Missing ⚠️
web/app/components/apps/index.tsx	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #35241      +/-   ##
==========================================
+ Coverage   84.85%   84.87%   +0.02%     
==========================================
  Files        4360     4363       +3     
  Lines      196669   196854     +185     
  Branches    37276    37323      +47     
==========================================
+ Hits       166883   167088     +205     
+ Misses      26706    26686      -20     
  Partials     3080     3080              

Flag	Coverage Δ
web	85.82% <96.62%> (+0.05%)	⬆️

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