feat(ui,roundtrip): modern neutral “Apple Glass” UI, scenic round trips, and isochrone guidance

- UI (neutral minimal, glass-like)
  - Rewrote app shell and control panel in [app/web/static/index.html](cci:7://file:///home/pedan/freemoto/app/web/static/index.html:0:0-0:0)
  - Modernized responsive styles in [app/web/static/styles.css](cci:7://file:///home/pedan/freemoto/app/web/static/styles.css:0:0-0:0)
  - Proper internal scrolling for control panel, suggestions, waypoints, and directions sheet
  - Improved dark mode, spacing, and visual polish

- Round Trip generation
  - Direction preference (N/E/S/W) with biased waypoint bearings ([route.js](cci:7://file:///home/pedan/freemoto/app/web/static/route.js:0:0-0:0))
  - Ferry avoidance by default on round trips; added strong ferry cost
  - Coastal safety: reverse-geocode water check and automatic inland adjustment for generated waypoints
  - New “Scenic optimizer” option: generate candidate loops, route them, score twistiness, pick the best
    - Heading-change/turn-density based scoring using decoded polyline

- Isochrone-guided loops
  - Backend: added `/isochrone` proxy in [app/web/main.go](cci:7://file:///home/pedan/freemoto/app/web/main.go:0:0-0:0) (derives Valhalla base from `VALHALLA_URL`)
  - Frontend: optional “Use isochrone guidance” + time (minutes); sample preferred sector of polygon

- Settings persistence and live updates
  - Direction, scenic, isochrone, and distance persist via localStorage
  - Changing options regenerates round trips when enabled

- Docker/Build
  - Confirmed multi-arch build (amd64/arm64) and push to git.ztsw.de/pedan/freemoto/freemoto-web:latest

Refs: [index.html](cci:7://file:///home/pedan/freemoto/app/web/static/index.html:0:0-0:0), [styles.css](cci:7://file:///home/pedan/freemoto/app/web/static/styles.css:0:0-0:0), [route.js](cci:7://file:///home/pedan/freemoto/app/web/static/route.js:0:0-0:0), [main.js](cci:7://file:///home/pedan/freemoto/app/web/static/main.js:0:0-0:0), [app/web/main.go](cci:7://file:///home/pedan/freemoto/app/web/main.go:0:0-0:0), [Dockerfile](cci:7://file:///home/pedan/freemoto/Dockerfile:0:0-0:0)

app/web/main.go

@@ -48,6 +48,54 @@ func parseLogLevel(s string) int {
     }
 }
 
+// proxyToValhallaPOST forwards a POST with JSON body to a specific Valhalla endpoint (e.g., /isochrone)
+func proxyToValhallaPOST(w http.ResponseWriter, r *http.Request, target string) {
+    if r.Method != http.MethodPost {
+        http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+        return
+    }
+    req, err := http.NewRequestWithContext(r.Context(), http.MethodPost, target, r.Body)
+    if err != nil {
+        http.Error(w, err.Error(), http.StatusBadRequest)
+        return
+    }
+    req.Header = r.Header.Clone()
+    if req.Header.Get("Content-Type") == "" {
+        req.Header.Set("Content-Type", "application/json")
+    }
+
+    client := http.Client{Timeout: 20 * time.Second}
+    start := time.Now()
+    resp, err := client.Do(req)
+    if err != nil {
+        if rid, ok := r.Context().Value(ctxReqID).(string); ok {
+            logf(levelError, "rid=%s upstream=valhalla target=%s error=%v", rid, target, err)
+        } else {
+            logf(levelError, "upstream=valhalla target=%s error=%v", target, err)
+        }
+        http.Error(w, err.Error(), http.StatusBadGateway)
+        return
+    }
+    defer resp.Body.Close()
+
+    // Forward common headers and status
+    for key, values := range resp.Header {
+        if key == "Content-Type" || key == "Content-Encoding" || key == "Cache-Control" || key == "Vary" {
+            for _, v := range values {
+                w.Header().Add(key, v)
+            }
+        }
+    }
+    w.WriteHeader(resp.StatusCode)
+    _, _ = io.Copy(w, resp.Body)
+
+    if rid, ok := r.Context().Value(ctxReqID).(string); ok {
+        logf(levelDebug, "rid=%s upstream=valhalla target=%s status=%d dur=%s", rid, target, resp.StatusCode, time.Since(start))
+    } else {
+        logf(levelDebug, "upstream=valhalla target=%s status=%d dur=%s", target, resp.StatusCode, time.Since(start))
+    }
+}
+
 func logf(level int, format string, args ...any) {
     if level < currentLogLevel {
         return
@@ -74,6 +122,8 @@ func main() {
     if valhallaURL == "" {
         valhallaURL = "http://valhalla:8002/route"
     }
+    // Derive Valhalla base for other endpoints like /isochrone
+    valhallaBase := strings.TrimSuffix(valhallaURL, "/route")
 
     nominatimBase := os.Getenv("NOMINATIM_URL")
     if nominatimBase == "" {
@@ -100,6 +150,15 @@ func main() {
     http.Handle("/route", withLogging(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
         proxyToValhalla(w, r, valhallaURL)
     })))
+    http.Handle("/isochrone", withLogging(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+        // Forward to Valhalla isochrone endpoint (POST JSON)
+        if r.Method != http.MethodPost {
+            http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+            return
+        }
+        target := valhallaBase + "/isochrone"
+        proxyToValhallaPOST(w, r, target)
+    })))
     // Nominatim proxy endpoints
     http.Handle("/geocode", withLogging(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
         proxyToNominatimGET(w, r, nominatimBase+"/search", nominatimUA)