Fix browse-by pages sending unnecessary request to 'author' index

[JorgeEscire]

References

Fixes #3572

Description

Removes hardcoded default initialization to 'author' browse index that caused unnecessary failed requests when visiting browse-by pages on systems where the 'author' index is disabled.

Instructions for Reviewers

List of changes in this PR:

• Removed hardcoded 'author' default from BrowseByMetadataComponent - the component now waits for route parameters before initializing browse requests
• Updated BrowseByDateComponent.ngOnInit() to properly use browse ID from route parameters instead of relying on parent's default
• Updated BrowseByTitleComponent.ngOnInit() to properly use browse ID from route parameters instead of relying on parent's default
• Fixed BrowseByDateComponent test spy to expect 'dateissued' instead of 'author'
• Improved loading state handling - loading$ properly initialized to show loading indicator while waiting for route parameters

Technical Details:

Before this fix:

• BrowseByMetadataComponent initialized with a hardcoded request to 'author' browse index
• When visiting any browse-by page (title, date, subject, etc.), two requests were made:
  1. First to /api/discover/browses/author/entries (unnecessary and would fail if disabled)
  2. Then to the correct browse index (e.g., /api/discover/browses/title/entries)

After this fix:

• Components wait for route parameters to be available
• Only one request is made to the correct browse index
• browseEntries$ remains undefined until properly initialized
• Loading indicator displays while parameters are being resolved

How to test this PR:

1. Start the application: npm start:dev
2. Open browser DevTools (F12) → Network tab
3. Filter by XHR/Fetch requests
4. Visit different browse-by pages:

• http://localhost:4000/browse/title
• http://localhost:4000/browse/dateissued
• http://localhost:4000/browse/subject

Expected behavior:

• Only ONE request should be made to the correct browse index
• NO request should be made to /api/discover/browses/author/entries
• Loading indicator should display briefly while route parameters are resolved
• Page should load successfully without errors

Checklist

• My PR is created against the main branch of code (unless it is a backport or is fixing an issue specific to an older branch).
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• My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
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• If my PR fixes an issue ticket, I've linked them together.