fix(work): return 400 when jobName is missing instead of defaulting

[Chzhqv]

Summary

• When the jobName query param was missing, the code searched user.workConfig.jobs for a job with name === null, which always returned undefined, then silently proceeded with hourlyRate=0 and the default weeklyTarget=20 instead of erroring — masking a missing parameter as if the user simply had no hours logged.
• Now returns 400 with a clear error when jobName is missing.

Test plan

• Verified live against the real dev server: before the fix, GET /api/work/summary with no jobName returned 200 with hourlyRate:0; after the fix it returns 400 {"error":"jobName is required"}
• Confirmed a valid jobName still resolves the real job config correctly (hourlyRate/weeklyTarget from the user's configured job)
• Confirmed a provided-but-nonexistent jobName keeps its existing fallback behavior (out of scope for this fix, not changed)
• pnpm lint, tsc --noEmit, full vitest run (556/556) all pass

Fixes #26

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