feat: add time-dependent Europe/Brussels dashboard greeting

The dashboard greeting was a fully static "Goedemorgen..." regardless of
actual time of day. New frontend/src/i18n/greeting.ts::getGreetingPeriod
is a pure, clock-injectable function resolving one of 4 periods (05:00-
11:59 morning, 12:00-17:59 afternoon, 18:00-22:59 evening, 23:00-04:59
night) against Europe/Brussels wall-clock time via
Intl.DateTimeFormat({ timeZone, hourCycle: "h23" }), which is DST-safe
by construction.

useGreetingPeriod.ts wires this into React with a 30s poll so the
greeting rolls over live while the app stays open, no reload required.
Each period now has its own greeting word and accompanying sentence in
all 3 languages (dashboard.json), replacing both the fixed "Goedemorgen"
and the fixed "Here's the fleet" follow-up sentence. Night never says
"Goedenacht" (used as a farewell, not a welcome, in Dutch).

frontend/src/i18n/useGreetingPeriod.ts

@@ -0,0 +1,23 @@
+import { useEffect, useState } from "react";
+import { getGreetingPeriod, type GreetingPeriod } from "./greeting";
+
+// Checked frequently enough that a period rollover (e.g. 11:59 -> 12:00) is picked up
+// within the Dashboard while the app stays open, without a page reload -- see section 9
+// of the Fleet Ops final localization brief.
+const CHECK_INTERVAL_MS = 30_000;
+
+export function useGreetingPeriod(): GreetingPeriod {
+  const [period, setPeriod] = useState<GreetingPeriod>(() => getGreetingPeriod());
+
+  useEffect(() => {
+    const id = setInterval(() => {
+      setPeriod((current) => {
+        const next = getGreetingPeriod();
+        return next === current ? current : next;
+      });
+    }, CHECK_INTERVAL_MS);
+    return () => clearInterval(id);
+  }, []);
+
+  return period;
+}