refactor(frontend): standardize menu.component subscription cleanup

[Ma77Ball]

What changes were proposed in this PR?

• Removed the redundant computingUnitStatusSubscription field (and its .add() wrapper and ngOnDestroy unsubscribe) in menu.component.ts: the inner stream is already piped through untilDestroyed(this), so the manual aggregator did nothing.
• Replaced the manually managed durationUpdateSubscription with a declarative switchMap pipe, so each ExecutionDurationUpdateEvent cancels the previous 1s timer and starts a new one only while running, instead of imperatively unsubscribing and reassigning a field.
• Dropped the now-unused Subscription import, leaving the component on a single consistent cleanup style (untilDestroyed).

Any related issues, documentation, or discussions?

Closes: #5323

How was this PR tested?

• Ran ng test --include='src/app/workspace/component/menu/menu.component.spec.ts': all 42 tests pass.
• Behavior-preserving refactor: same elapsed-timer cadence, same base-duration updates, same teardown on destroy.

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

Co-authored with Claude Opus 4.7 in compliance with ASF

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.31%. Comparing base (8a4cb29) to head (00f7b8a).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5324      +/-   ##
============================================
- Coverage     51.62%   51.31%   -0.31%     
+ Complexity     2420     2385      -35     
============================================
  Files          1056     1053       -3     
  Lines         41046    40515     -531     
  Branches       4407     4317      -90     
============================================
- Hits          21190    20791     -399     
+ Misses        18617    18525      -92     
+ Partials       1239     1199      -40     

Flag	Coverage Δ		*Carryforward flag
access-control-service	41.89% <ø> (ø)		Carriedforward from 34be37d
agent-service	33.76% <ø> (ø)		Carriedforward from 34be37d
amber	51.65% <ø> (-0.58%)	⬇️	Carriedforward from 34be37d
computing-unit-managing-service	0.00% <ø> (-1.39%)	⬇️	Carriedforward from 34be37d
config-service	0.00% <ø> (-54.69%)	⬇️	Carriedforward from 34be37d
file-service	38.42% <ø> (ø)		Carriedforward from 34be37d
frontend	46.30% <100.00%> (-0.03%)	⬇️
pyamber	?
python	90.84% <ø> (ø)		Carriedforward from 34be37d
workflow-compiling-service	57.25% <ø> (-1.15%)	⬇️	Carriedforward from 34be37d

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

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[Yicong-Huang]

@Ma77Ball please add tests to verify the described issue does not happen.

[Ma77Ball]

/request-review @Yicong-Huang