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/de.json

@@ -1667,6 +1667,11 @@
   "TomTom roads first; OpenStreetMap fills the rest, simulated": "Zuerst TomTom-Straßen; OpenStreetMap füllt den Rest, simuliert",
   "TomTom roads first; OpenStreetMap fills the rest, simulated (needs a TomTom key)": "Zuerst TomTom-Straßen; OpenStreetMap füllt den Rest, simuliert (braucht einen TomTom-Schlüssel)",
   "Hybrid": "Hybrid",
+  "Lines": "Linien",
+  "Dots": "Punkte",
+  "Road lines colored by live flow: green to red": "Straßenlinien nach Live-Verkehrsfluss gefärbt: grün bis rot",
+  "Moving dots colored by live flow": "Bewegte Punkte nach Live-Verkehrsfluss gefärbt",
+  "Moving dots": "Bewegte Punkte",
   "TomTom key unavailable": "TomTom-Schlüssel nicht verfügbar",
   "TomTom daily budget reached": "TomTom-Tagesbudget erreicht",
   "Monthly traffic allowance used": "Monatliches Verkehrskontingent aufgebraucht",