Resource control: make UI polling/backoff limits configurable

[mark-e-deyoung]

Summary

Make dashboard polling/backoff constants runtime-configurable with safe defaults.

Problem

UI uses fixed constants for polling/backoff windows. These are reasonable defaults but not tunable for high-latency or constrained deployments.

Scope

• Add configuration delivery path (e.g., handshake/config endpoint) for UI timing defaults.
• Make these configurable with validated ranges:
  • base poll delay
  • backoff max elapsed
  • backoff max step delay
  • failure threshold before entering backoff
• Preserve current defaults if config is absent.
• Add UI tests for applied config behavior.

Acceptance criteria

• Operators can tune UI polling/backoff without editing source.
• Defaults remain unchanged when no overrides provided.
• UI remains stable under extreme but valid settings.