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/greeting.ts

@@ -0,0 +1,28 @@
+export type GreetingPeriod = "morning" | "afternoon" | "evening" | "night";
+
+const BRUSSELS_TIME_ZONE = "Europe/Brussels";
+
+// DST-safe: Intl.DateTimeFormat resolves the correct Europe/Brussels wall-clock hour for
+// any instant, automatically accounting for the CET/CEST transition -- no manual UTC
+// offset math (which would silently break twice a year) is needed. `hourCycle: "h23"`
+// pins the output to a plain 0-23 range (some engines otherwise render midnight as "24").
+export function getBrusselsHour(date: Date): number {
+  const formatter = new Intl.DateTimeFormat("en-GB", {
+    timeZone: BRUSSELS_TIME_ZONE,
+    hour: "numeric",
+    hourCycle: "h23",
+  });
+  return Number(formatter.format(date));
+}
+
+// Boundaries per the Fleet Ops final localization brief (section 9):
+//   05:00-11:59 morning, 12:00-17:59 afternoon, 18:00-22:59 evening, 23:00-04:59 night.
+// `date` defaults to `new Date()` but accepts any Date so callers (and tests) can inject
+// a fixed clock instead of depending on the real wall clock.
+export function getGreetingPeriod(date: Date = new Date()): GreetingPeriod {
+  const hour = getBrusselsHour(date);
+  if (hour >= 5 && hour < 12) return "morning";
+  if (hour >= 12 && hour < 18) return "afternoon";
+  if (hour >= 18 && hour < 23) return "evening";
+  return "night";
+}

frontend/src/i18n/locales/en-GB/dashboard.json

@@ -1,6 +1,17 @@
 {
   "eyebrow": "Operations / Live overview",
-  "title": "Good morning. Here's the fleet.",
+  "greeting": {
+    "morning": "Good morning",
+    "afternoon": "Good afternoon",
+    "evening": "Good evening",
+    "night": "Welcome back"
+  },
+  "greetingBody": {
+    "morning": "Here's today's fleet status.",
+    "afternoon": "Here's the current overview of your fleet.",
+    "evening": "Here's this evening's fleet overview.",
+    "night": "Here's the latest overview of your fleet."
+  },
   "description": "Readiness, exceptions and hand-offs across today's operation.",
   "viewFleet": "View fleet",
   "demoStart": {

frontend/src/i18n/locales/fr-BE/dashboard.json

@@ -1,6 +1,17 @@
 {
   "eyebrow": "Exploitation / Aperçu en direct",
-  "title": "Bonjour. Voici votre flotte.",
+  "greeting": {
+    "morning": "Bonjour",
+    "afternoon": "Bonjour",
+    "evening": "Bonsoir",
+    "night": "Bon retour"
+  },
+  "greetingBody": {
+    "morning": "Voici l'état de votre flotte pour aujourd'hui.",
+    "afternoon": "Voici l'aperçu actuel de votre flotte.",
+    "evening": "Voici l'aperçu de votre flotte pour ce soir.",
+    "night": "Voici le dernier aperçu de votre flotte."
+  },
   "description": "Disponibilité, exceptions et transferts de l'activité en cours.",
   "viewFleet": "Voir la flotte",
   "demoStart": {

frontend/src/i18n/locales/nl-BE/dashboard.json

@@ -1,6 +1,17 @@
 {
   "eyebrow": "Uitvoeren / Live overzicht",
-  "title": "Goedemorgen. Hier is je wagenpark.",
+  "greeting": {
+    "morning": "Goedemorgen",
+    "afternoon": "Goedemiddag",
+    "evening": "Goedenavond",
+    "night": "Welkom terug"
+  },
+  "greetingBody": {
+    "morning": "Hier is de status van je wagenpark voor vandaag.",
+    "afternoon": "Hier is het actuele overzicht van je wagenpark.",
+    "evening": "Hier is het overzicht van je wagenpark voor vanavond.",
+    "night": "Hier is het laatste overzicht van je wagenpark."
+  },
   "description": "Beschikbaarheid, uitzonderingen en overdrachten doorheen de huidige werking.",
   "viewFleet": "Wagenpark bekijken",
   "demoStart": {

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;
+}