fix(abs): show Temporarily Unavailable on 5xx instead of Record Not Found

[thostetler]

When the upstream search API returns a 5xx during SSR for an abstract
page, users see "Record Not Found" — the same message as a genuinely
missing record. This is misleading; the record exists but the service
was temporarily unavailable.

• Added statusCode to SSR error props in absCanonicalization.ts
• Created ServiceUnavailable component with retry/back actions
• Updated all 10 abstract pages to show ServiceUnavailable for 5xx
• RecordNotFound still shown for actual missing records (no statusCode or 4xx)

[codecov]

Codecov Report

❌ Patch coverage is 94.36620% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.6%. Comparing base (968d8a6) to head (12eb257).

Files with missing lines	Patch %	Lines
src/lib/serverside/absCanonicalization.ts	25.0%	3 Missing ⚠️
...mponents/ServiceUnavailable/ServiceUnavailable.tsx	98.6%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #800     +/-   ##
========================================
+ Coverage    61.6%   61.6%   +0.1%     
========================================
  Files         312     313      +1     
  Lines       36000   36070     +70     
  Branches     1594    1597      +3     
========================================
+ Hits        22148   22213     +65     
- Misses      13815   13820      +5     
  Partials       37      37             

Files with missing lines	Coverage Δ
...mponents/ServiceUnavailable/ServiceUnavailable.tsx	98.6% <98.6%> (ø)
src/lib/serverside/absCanonicalization.ts	62.7% <25.0%> (-0.4%)	⬇️

... and 1 file with indirect coverage changes

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