Show time-remaining countdown on stream card (#33)

Summary

This PR improves the readability of Stream cards by replacing the raw date range display with a relative countdown showing the remaining time until a stream ends.

Instead of displaying dates such as:

Jun 3 2026 → Jun 10 2026

the card now displays:

• 2 days left
• 5 hours left
• 30 minutes left
• Ended

The original absolute date range remains accessible through a tooltip on hover.

Changes Made

1. Added formatTimeRemaining utility

File: src/lib/format.ts

Implemented a new helper:

formatTimeRemaining(endSec: number, nowSec?: number): string

Features:

• Returns the largest sensible time unit remaining (days, hours, minutes, or seconds).
• Handles singular and plural forms correctly.
• Returns "Ended" when the current time is equal to or greater than endSec.
• Supports an optional nowSec parameter for deterministic testing.

2. Updated Stream Card UI

File: src/components/StreamCard.tsx

• Replaced the raw date range text with formatTimeRemaining.
• Reused the existing ended status logic for completed/cancelled streams.
• Added a title attribute containing the full date range so exact dates remain available on hover.

3. Added Frontend Unit Testing

Files:

• package.json
• vitest.config.ts
• src/lib/format.test.ts

Changes include:

• Added Vitest as a development dependency.

• Added a test script for running unit tests.

• Configured Vitest with a Node test environment.

• Added unit tests covering:

  • Days remaining
  • Hours remaining
  • Minutes remaining
  • Seconds remaining
  • Ended streams
  • Default nowSec behavior

Verification

Unit Tests

pnpm test

Result:

✓ src/lib/format.test.ts (6 tests)
Test Files  1 passed (1)
Tests       6 passed (6)

Type Checking

pnpm exec tsc --noEmit

Result:

• ✅ Passes with no TypeScript errors.

Acceptance Criteria

• ✅ Active streams display a relative countdown.
• ✅ Past streams display "Ended".
• ✅ Exact dates remain accessible via tooltip.
• ✅ formatTimeRemaining includes unit tests.
• ✅ pnpm exec tsc --noEmit passes.