Fix shift hours overcounting automatic breaks + add version-check banner

Workforce's automatic_break_length is a payroll deduction setting, not a
scheduling field — applying it when no explicit breaks are present was silently
shortening all shifts (e.g. a 2h shift showing as 1.5h). Now only deducts
explicit break records from s.breaks. Also adds BUILD_VERSION to /health and
an UpdateBanner that prompts staff to reload when a new deploy lands.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/hooks/useVersionCheck.ts

@@ -0,0 +1,43 @@
+import { useEffect, useState } from 'react'
+
+const POLL_MS = 2 * 60 * 1000
+
+export function useVersionCheck(healthUrl: string) {
+  const [updateAvailable, setUpdateAvailable] = useState(false)
+
+  useEffect(() => {
+    let seenVersion: string | null = null
+
+    async function check() {
+      try {
+        const res = await fetch(healthUrl, { cache: 'no-store' })
+        if (!res.ok) return
+        const data = await res.json()
+        const v: string | undefined = data.version
+        if (!v) return
+        if (seenVersion === null) {
+          seenVersion = v
+        } else if (v !== seenVersion) {
+          setUpdateAvailable(true)
+        }
+      } catch {
+        // network error — skip silently
+      }
+    }
+
+    check()
+    const interval = setInterval(check, POLL_MS)
+
+    function onVisible() {
+      if (document.visibilityState === 'visible') check()
+    }
+    document.addEventListener('visibilitychange', onVisible)
+
+    return () => {
+      clearInterval(interval)
+      document.removeEventListener('visibilitychange', onVisible)
+    }
+  }, [healthUrl])
+
+  return updateAvailable
+}