feat(traffic): Lines and Dots toggles; hosts choose which road modes they offer

createTrafficSource({ roadModes }) lets a host offer a subset of the TomTom,
OSM and Hybrid road modes (all three by default). Without OSM the layer never
draws simulated roads, and a single mode shows no road chip.

While flow is live the Street Traffic row offers Lines (every road with flow
as a steady green/amber/red line, from the heat-line renderer) and Dots (the
moving dots), saved in layer state and share links. Defaults: Lines off,
Dots on.

src/i18n/locales/it.json

@@ -1103,6 +1103,11 @@
   "TomTom roads first; OpenStreetMap fills the rest, simulated": "Prima le strade TomTom; OpenStreetMap completa il resto, simulato",
   "TomTom roads first; OpenStreetMap fills the rest, simulated (needs a TomTom key)": "Prima le strade TomTom; OpenStreetMap completa il resto, simulato (richiede una chiave TomTom)",
   "Hybrid": "Ibrido",
+  "Lines": "Linee",
+  "Dots": "Punti",
+  "Road lines colored by live flow: green to red": "Strade colorate in base al traffico in tempo reale: dal verde al rosso",
+  "Moving dots colored by live flow": "Punti in movimento colorati in base al traffico in tempo reale",
+  "Moving dots": "Punti in movimento",
   "TomTom key unavailable": "Chiave TomTom non disponibile",
   "TomTom daily budget reached": "Budget giornaliero TomTom esaurito",
   "Monthly traffic allowance used": "Quota mensile del traffico esaurita",