feat: add treasury-balance-derived at-risk health state to StreamDetail sidebar (Closes #1280)

[waterWang]

Closes #1280

Description

Add a treasury-balance-derived "At risk" funding health state to the StreamDetail sidebar. The existing health pill reflects the stream's own status (active/paused/completed); this adds a distinct, layered indicator that surfaces when the remaining balance only covers under 48 hours of accrual at the stream's monthly rate.

Changes

• src/pages/StreamDetail.tsx

  • Added AT_RISK_RUNWAY_HOURS = 48 named constant (single, tunable threshold).
  • Added HOURS_PER_MONTH = 720 (30-day month) for the runway calculation.
  • Added computeFundingRunwayHours(remainingAmount, monthlyRate) — returns Infinity when the monthly rate is zero (no accrual → no funding risk) and 0 when the remaining balance is exhausted.
  • Added isFundingAtRisk(stream) — true when the computed runway is below the 48-hour threshold, and excludes completed streams (already settled).
  • Rendered an "At risk" badge (alert-triangle icon + label, not color alone) next to the existing health pill when isFundingAtRisk is true. It uses design tokens (--color-error-subtle, --color-error, --color-error-border), is role="alert" for screen readers, and is responsive (inline-flex, wraps alongside the pill).

• src/pages/__tests__/StreamDetail.edgeCases.test.tsx

  • Unit tests for computeFundingRunwayHours (happy path, zero/negative rate, zero/negative balance).
  • Unit tests for isFundingAtRisk (below/above threshold, completed streams, zero rate).
  • Integration tests asserting the at-risk badge renders when runway < 48h, and does not render for sufficient runway or completed streams.

Verification

• New tests pass: 12/12 (vitest run ... -t "Funding").
• tsc --noEmit reports no new errors in StreamDetail.tsx / its tests.
• The threshold is a single named constant so it can be tuned without hunting through the component.
• WCAG 2.1 AA: the badge is announced via role="alert" and relies on an icon + label (not color alone).

Screenshots / a11y notes

• At-risk badge: red-tinted pill with an alert-triangle icon and "Funding at risk — less than 48h of runway" text, shown inline next to the stream's health pill.
• Non-at-risk streams: unchanged (no badge).