Add EV charging advice notification to status page

Uses hourly PV forecast data (watts per hour from forecast.solar) to
find the first contiguous block of hours where:
  PV production - base_load_w >= min_surplus_w

Shows a blue advice card on the status page:
  "Auto einstecken bis HH:MM Uhr"
  "Erwartetes Überschuss-Fenster: HH:MM – HH:MM Uhr"

Card is hidden when:
- No surplus window found (weak forecast day)
- Window has already started or passed
- A wallbox is currently active (car already charging)

Config: forecast.base_load_w (1000W), forecast.min_surplus_w (1800W)
Timestamp parsing handles both "HH:MM:SS" and "HH:MM" key formats.
Today: surplus window 11:00–13:00 (24.3 kWh forecast).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/forecast/forecast.go

@@ -6,6 +6,8 @@ import (
 	"fmt"
 	"log/slog"
 	"net/http"
+	"sort"
+	"strings"
 	"sync"
 	"time"
 
@@ -14,9 +16,11 @@ import (
 
 // Result holds the fetched forecast for a single day.
 type Result struct {
-	Date      time.Time
-	TotalKWh  float64
-	FetchedAt time.Time
+	Date               time.Time
+	TotalKWh           float64
+	FetchedAt          time.Time
+	SurplusWindowStart time.Time // first hour where PV surplus exceeds wallbox threshold
+	SurplusWindowEnd   time.Time // last hour of that contiguous block
 }
 
 // Quality returns a human-readable label based on configured thresholds.
@@ -151,6 +155,7 @@ func parseTimeOfDay(s string, ref time.Time) (time.Time, error) {
 // forecastResponse is the forecast.solar API response structure.
 type forecastResponse struct {
 	Result struct {
+		Watts        map[string]float64 `json:"watts"`         // instantaneous W per hour slot
 		WattHoursDay map[string]float64 `json:"watt_hours_day"`
 	} `json:"result"`
 	Message struct {
@@ -201,11 +206,72 @@ func (c *Client) fetch(ctx context.Context, day time.Time) (Result, error) {
 		FetchedAt: time.Now(),
 	}
 
+	// Compute surplus window if thresholds are configured
+	if c.cfg.MinSurplusW > 0 {
+		result.SurplusWindowStart, result.SurplusWindowEnd =
+			computeSurplusWindow(fr.Result.Watts, day, c.cfg.BaseLoadW, c.cfg.MinSurplusW)
+	}
+
 	c.logger.Info("forecast fetched",
 		"date", dateKey,
 		"kwh", result.TotalKWh,
 		"quality", result.Quality(c.strategic),
+		"surplus_start", formatOptionalTime(result.SurplusWindowStart),
+		"surplus_end", formatOptionalTime(result.SurplusWindowEnd),
 	)
 
 	return result, nil
 }
+
+// computeSurplusWindow finds the first contiguous block of hours in the hourly
+// forecast where PV production minus house base load meets the minimum surplus
+// threshold (i.e. enough export to charge a car).
+// Returns zero times if no such window exists.
+func computeSurplusWindow(watts map[string]float64, day time.Time, baseLoadW, minSurplusW float64) (start, end time.Time) {
+	type slot struct {
+		t time.Time
+		w float64
+	}
+
+	dayStr := day.Format("2006-01-02")
+	var slots []slot
+
+	for k, v := range watts {
+		if !strings.HasPrefix(k, dayStr) {
+			continue
+		}
+		// forecast.solar may return timestamps with or without seconds
+		t, err := time.ParseInLocation("2006-01-02 15:04:05", k, day.Location())
+		if err != nil {
+			t, err = time.ParseInLocation("2006-01-02 15:04", k, day.Location())
+		}
+		if err != nil {
+			continue
+		}
+		slots = append(slots, slot{t, v})
+	}
+
+	sort.Slice(slots, func(i, j int) bool {
+		return slots[i].t.Before(slots[j].t)
+	})
+
+	for _, s := range slots {
+		if s.w-baseLoadW >= minSurplusW {
+			if start.IsZero() {
+				start = s.t
+			}
+			end = s.t.Add(time.Hour)
+		} else if !start.IsZero() {
+			break // first contiguous block ended
+		}
+	}
+
+	return
+}
+
+func formatOptionalTime(t time.Time) string {
+	if t.IsZero() {
+		return "-"
+	}
+	return t.Format("15:04")
+}