SCIX-794, SCIX-774: Add discipline routes, fix legacy referrer, rename General Science

[thostetler]

Replaces the deprecated /onboard route with discipline-specific routes (/astrophysics, /heliophysics, /planetary, /earth, /biophysical) that immediately set the selected discipline and redirect to home.

Also fixes the bug where users arriving from the legacy ADS app (ui.adsabs.harvard.edu) don't immediately see the Astrophysics discipline selected - it previously only switched on the next search.

Additionally renames "General Science" to "No preferred discipline" (SCIX-774).

• Add mapPathToDisciplineParam utility for extracting discipline from URL paths
• Add middleware redirect for discipline routes to /?forceMode=X
• Honor forceMode query param in SSR with highest precedence
• Redirect legacy ADS referrer to /?forceMode=astrophysics in middleware
• Remove deprecated /onboard route and its noindex header config
• Clean forceMode param from URL after mode is applied
• Add tests for utility and SSR forceMode handling
• Rename "General Science" label to "No preferred discipline"

[codecov]

Codecov Report

❌ Patch coverage is 80.35714% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.9%. Comparing base (5d0ff6f) to head (15d8f4f).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/middleware.ts	62.1%	11 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #773     +/-   ##
========================================
+ Coverage    60.9%   60.9%   +0.1%     
========================================
  Files         301     301             
  Lines       35040   35081     +41     
  Branches     1500    1507      +7     
========================================
+ Hits        21320   21349     +29     
- Misses      13686   13698     +12     
  Partials       34      34             

Files with missing lines	Coverage Δ
src/components/NavBar/models.ts	100.0% <100.0%> (ø)
src/ssr-utils.ts	70.0% <100.0%> (+2.5%)	⬆️
src/utils/appMode.ts	86.3% <100.0%> (+1.8%)	⬆️
src/middleware.ts	91.8% <62.1%> (-2.0%)	⬇️

... and 1 file with indirect coverage changes

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