[SandObserver]

Summary

• Add ui/js/jitter.js, holding the ±15% spread as one dependency-free helper, and register both path forms in tsconfig.frontend.json.
• Use it for the repeat interval in the widget toolbox's poll(), leaving the first fetch immediate.
• Replace the dashboard shell's local _jit with the shared helper, so the rule has one definition.
• Return 0 for an interval that is missing, negative or not a number, so a caller cannot schedule a NaN delay that setTimeout turns into a busy loop.
• Document the behaviour on poll() in docs/widgets.md.

Testing
All green, apart from the Docker build smoke test, which needs a local Docker daemon. Added 5 tests: the jitter bounds under stubbed randomness and across 500 draws, unusable intervals, and a poll test that captures the scheduled delay to confirm the repeat is spread while the first fetch waits for nothing.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.