CUB-196: fix future timestamps, negative battery clamp, boundary tests

Grimm review fixes:
- formatRelativeTime: guard future timestamps (clock skew) → 'unknown'
- battery display: clamp negative values to 0%
- formatTimeLeft: floor fractional seconds
- Tests: +4 (future timestamp, negative battery, 15%/50% boundaries)

src/components/CameraCard.test.tsx

@@ -131,4 +131,33 @@ describe('CameraCard', () => {
     )
     expect(screen.getByText('unknown')).toBeInTheDocument()
   })
+
+  it('shows "unknown" when last_seen is in the future', () => {
+    const future = new Date(Date.now() + 86400000).toISOString() // +1 day
+    render(<CameraCard camera={makeCamera({ last_seen: future })} />)
+    expect(screen.getByText('unknown')).toBeInTheDocument()
+  })
+
+  // ── Edge cases ──────────────────────────────────────────────────────────
+
+  it('clamps negative battery_pct to 0%', () => {
+    render(<CameraCard camera={makeCamera({ battery_pct: -5 })} />)
+    expect(screen.getByText('0%')).toBeInTheDocument()
+  })
+
+  it('shows exact boundary: 15% battery → yellow bar', () => {
+    const { container } = render(
+      <CameraCard camera={makeCamera({ battery_pct: 15 })} />,
+    )
+    const bar = container.querySelector('[role="progressbar"] div')
+    expect(bar?.className).toContain('bg-rig-warning')
+  })
+
+  it('shows exact boundary: 50% battery → green bar', () => {
+    const { container } = render(
+      <CameraCard camera={makeCamera({ battery_pct: 50 })} />,
+    )
+    const bar = container.querySelector('[role="progressbar"] div')
+    expect(bar?.className).toContain('bg-rig-success')
+  })
 })