Improve cron delivery warnings and error feedback

[webup]

What

• add a warning in the cron editor when no channel accounts are configured
• stop surfacing raw structured cron run output as broken success text like {
• make the visible cron status badge open run history directly when users need failure details
• add focused adapter and page tests for the new cron behavior

Why

• #143 reported that cron creation gave no delivery warning when the workspace had no configured message channels
• #145 reported that Run Now could show an unusable success banner and that visible error badges did not explain failures
• these are small but high-friction UX problems on a workflow users touch directly from the web UI

How

• derive configured channel-account count from the loaded config and show a non-blocking warning in the cron editor
• parse structured cron run stdout more defensively and fall back to the generic localized success message when no human-readable message is present
• turn the visible lastStatus pill into an actionable control that opens run history for that job
• cover the new behavior with focused Vitest coverage in the cron page and adapter suites

Testing

• npx vitest run src/shared/adapters/__tests__/cron.test.ts src/modules/cron/__tests__/CronPage.test.tsx
• npm run build
• verified with dev-browser on /cron that the no-channel warning renders in the editor and clicking the visible error pill opens run history

Resolves #143
Resolves #145