Fix den-web API proxy base resolution

[devcool20]

Summary

• Derive Den API requests through den-web's /api/den proxy when only a web/control-plane URL is configured.
• Repair stale settings where apiBaseUrl was stored equal to baseUrl.
• Preserve explicit separate Den API hosts such as https://den-api-example.run.app.

Issue

Closes #1808

Testing

• git diff --check - passed
• Resolver validation with bun -e - passed
• Manual Electron validation - passed

pnpm typecheck is currently blocked by an unrelated upstream dev error:

src/react-app/shell/command-palette.tsx(31,15): error TS2304: Cannot find name 'MutableRefObject'.
<img width="1251" height="180" alt="image" src="https://github.com/user-attachments/assets/b8960eaf-bb81-48b2-b491-6c2c4a739b84" />

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Project	Deployment	Actions	Updated (UTC)
openwork-landing	Ready	Preview, Comment, Open in v0	May 15, 2026 7:53pm

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[devcool20]

I verified the typecheck failure locally and it appears unrelated to this PR. The error is in src/react-app/shell/command-palette.tsx, while this PR only changes apps/app/src/app/lib/den.ts.

Focused validation for this change passes: git diff --check, resolver validation, and manual Electron validation for /api/den URL resolution.