feat(gui): add a toggle for Texera attribution

[xuang7]

What changes were proposed in this PR?

This PR adds an attribution toggle for downstream projects to display a "Powered by Texera" link in the sidebar.

Changes:

• Controlled by gui.attribution-enabled (env: GUI_ATTRIBUTION_ENABLED), default false.
• Hidden automatically when the sidebar is collapsed.
• Links to https://texera.io.

Demo:

Any related issues, documentation, discussions?

Closes #5038

How was this PR tested?

Existing automated tests pass and manually tested.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Opus 4.7

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 42.90%. Comparing base (a820f67) to head (33c7f57).

Files with missing lines	Patch %	Lines
...pache/texera/service/resource/ConfigResource.scala	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5039      +/-   ##
============================================
- Coverage     43.12%   42.90%   -0.22%     
  Complexity     2208     2208              
============================================
  Files          1045     1045              
  Lines         40249    40175      -74     
  Branches       4252     4253       +1     
============================================
- Hits          17358    17239     -119     
- Misses        21817    21862      +45     
  Partials       1074     1074              

Flag	Coverage Δ		*Carryforward flag
access-control-service	39.53% <ø> (ø)
agent-service	33.72% <ø> (ø)		Carriedforward from bb795a0
amber	43.76% <ø> (+<0.01%)	⬆️
computing-unit-managing-service	0.00% <ø> (ø)
config-service	0.00% <0.00%> (ø)
file-service	32.18% <ø> (ø)
frontend	34.08% <100.00%> (+0.02%)	⬆️
python	89.07% <ø> (-1.35%)	⬇️	Carriedforward from bb795a0
workflow-compiling-service	56.81% <ø> (ø)

*This pull request uses carry forward flags. Click here to find out more.

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[aicam]

LGTM