fix: harden desktop send response parsing

[KyleAMathews]

Summary

• preserve configured auth headers when local Electron requests fall back from desktop IPC transport to browser fetch
• replace raw res.json() calls in desktop send/session mutation paths with optional/required JSON helpers
• add a regression test for local Electron FormData fallback header injection

Testing

• pnpm --filter @electric-ax/agents-server-ui test -- --run src/lib/auth-fetch.test.ts

[github-actions]

Electric Agents Desktop Builds

Build artifacts for commit 23fc215.

Platform	Status	Artifact
macOS Apple Silicon	Passed	DMG
macOS Intel	Passed	DMG
Windows x64	Passed	Installer
Linux x64	Passed	AppImage / deb

Workflow run

[codecov]

Codecov Report

❌ Patch coverage is 11.76471% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.37%. Comparing base (cd8c467) to head (23fc215).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...gents-server-ui/src/lib/ElectricAgentsProvider.tsx	0.00%	26 Missing ⚠️
packages/agents-server-ui/src/lib/sendMessage.ts	0.00%	19 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           require-pg-sync-url    #4575      +/-   ##
=======================================================
+ Coverage                56.74%   58.37%   +1.62%     
=======================================================
  Files                      327      370      +43     
  Lines                    38000    40763    +2763     
  Branches                 10953    11576     +623     
=======================================================
+ Hits                     21564    23795    +2231     
- Misses                   16401    16894     +493     
- Partials                    35       74      +39     

Flag	Coverage Δ
packages/agents	71.36% <ø> (ø)
packages/agents-mcp	77.70% <ø> (?)
packages/agents-mobile	75.49% <ø> (ø)
packages/agents-runtime	82.48% <ø> (-0.02%)	⬇️
packages/agents-server	75.05% <ø> (+0.02%)	⬆️
packages/agents-server-ui	6.29% <11.76%> (+0.04%)	⬆️
packages/electric-ax	46.42% <ø> (ø)
packages/experimental	87.73% <ø> (?)
packages/react-hooks	86.48% <ø> (?)
packages/start	82.83% <ø> (?)
packages/typescript-client	91.71% <ø> (?)
packages/y-electric	56.05% <ø> (?)
typescript	58.37% <11.76%> (+1.62%)	⬆️
unit-tests	58.37% <11.76%> (+1.62%)	⬆️

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[github-actions]

Electric Agents Mobile Build

Local mobile checks ran for commit 23fc215.

The EAS Android preview build was skipped because the mobile-eas-build label is not present.
Add the mobile-eas-build label to this PR to produce an installable preview build.

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