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/fr.json

@@ -1103,6 +1103,11 @@
   "TomTom roads first; OpenStreetMap fills the rest, simulated": "Routes TomTom d'abord ; OpenStreetMap complète le reste, en simulation",
   "TomTom roads first; OpenStreetMap fills the rest, simulated (needs a TomTom key)": "Routes TomTom d'abord ; OpenStreetMap complète le reste, en simulation (nécessite une clé TomTom)",
   "Hybrid": "Hybride",
+  "Lines": "Lignes",
+  "Dots": "Points",
+  "Road lines colored by live flow: green to red": "Routes colorées selon le trafic en direct : du vert au rouge",
+  "Moving dots colored by live flow": "Points mobiles colorés selon le trafic en direct",
+  "Moving dots": "Points mobiles",
   "TomTom key unavailable": "Clé TomTom indisponible",
   "TomTom daily budget reached": "Quota quotidien TomTom atteint",
   "Monthly traffic allowance used": "Quota mensuel de trafic épuisé",