fix(dataset): load cover image on detail page for private datasets

[xuang7]

What changes were proposed in this PR?

This PR fixes the cover image failing to load on the dataset detail page for private datasets, and removes the default placeholder shown when no cover image is set.

Changes:

• Adds a JWT-aware endpoint GET /dataset/{did}/cover-url that returns the presigned S3 URL as JSON. The detail page calls it via Angular HttpClient (which carries the JWT), then sets the returned URL on <img src> (presigned URL needs no auth).
• Removes the meaningless default blue placeholder on the detail page: if no cover image is set, nothing is rendered.
• The existing 307-redirect endpoint /cover is preserved for the hub browse page, which serves only public datasets and consumes the redirect directly. Both endpoints now have doc comments cross-referencing each other.

Demo:
With cover image:

Without cover image:

Any related issues, documentation, discussions?

Closes #5283

How was this PR tested?

Added test cases for the new endpoint.
Manually verified on a local instance.

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

Generated-by: Claude Opus 4.7

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 49.12%. Comparing base (ec12c88) to head (ab8b5eb).

Files with missing lines	Patch %	Lines
.../dashboard/service/user/dataset/dataset.service.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #5284   +/-   ##
=========================================
  Coverage     49.12%   49.12%           
- Complexity     2378     2379    +1     
=========================================
  Files          1051     1051           
  Lines         40348    40350    +2     
  Branches       4279     4279           
=========================================
+ Hits          19821    19822    +1     
- Misses        19368    19369    +1     
  Partials       1159     1159           

Flag	Coverage Δ		*Carryforward flag
access-control-service	41.89% <ø> (ø)
agent-service	33.76% <ø> (ø)		Carriedforward from ec12c88
amber	51.56% <ø> (ø)		Carriedforward from ec12c88
computing-unit-managing-service	0.00% <ø> (ø)
config-service	0.00% <ø> (ø)
file-service	38.42% <100.00%> (+0.42%)	⬆️
frontend	41.07% <0.00%> (-0.01%)	⬇️
python	90.79% <ø> (ø)		Carriedforward from ec12c88
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!