Added TypeScript for frontend

Added type definitions for components, functions, data
Added guards for network responses
fixes #8

front/src/components/LineDot.tsx

@@ -0,0 +1,16 @@
+import { colors, lineNames } from '../assets/metro'
+import { lineByName } from '../utils/metro'
+
+function LineDot({ station }: { station: string }) {
+    const line = lineByName(station)
+
+    if (line == undefined)
+        return <></>
+
+    const lineTitle = lineNames[line]
+    const color = colors[line]
+
+    return <span title={`${lineTitle} ветка`} style={{ color: color }}>&#11044;</span>
+}
+
+export default LineDot