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/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": {