SCIX-831 fix(nav): replace return-to-results with router.back()

[thostetler]

The "Return to results" button used latestQuery from Zustand to reconstruct a search URL. This broke when users navigated to an abstract from a non-search context (e.g. a library document list) — the button would route back to a stale earlier search instead of the actual page they came from.

• Replaced useBackToSearchResults with a minimal hook returning router.back directly
• Updated all four consumers: SearchBar, authoraffiliations, citation_helper, exportcitation
• Relabeled button from "Return to results" to "Go back" (destination is now context-dependent)
• Removed the latestQuery.q !== '' visibility guard — back button always shows on pages that use it
• exportcitation preserves SimpleLink when an explicit referrer URL is passed from the library
• New test file for useBackToSearchResults

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.0%. Comparing base (2c0910b) to head (16aadc1).

Files with missing lines	Patch %	Lines
src/components/SearchBar/SearchBar.tsx	20.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #812     +/-   ##
========================================
+ Coverage    62.0%   62.0%   +0.1%     
========================================
  Files         317     316      -1     
  Lines       36536   36423    -113     
  Branches     1642    1642             
========================================
- Hits        22621   22570     -51     
+ Misses      13878   13816     -62     
  Partials       37      37             

Files with missing lines	Coverage Δ
src/lib/useBackToSearchResults.ts	100.0% <100.0%> (+8.4%)	⬆️
src/components/SearchBar/SearchBar.tsx	67.2% <20.0%> (ø)

... and 2 files with indirect coverage changes

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