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/config/config.go

@@ -131,12 +131,14 @@ type ForecastConfig struct {
 	Enabled          bool    `yaml:"enabled"`
 	Lat              float64 `yaml:"lat"`
 	Lon              float64 `yaml:"lon"`
-	Declination      int     `yaml:"declination"`   // panel tilt in degrees
-	Azimuth          int     `yaml:"azimuth"`        // degrees from south (south=0, west=90)
-	KWp              float64 `yaml:"kwp"`            // installed peak power
-	FetchInterval    string  `yaml:"fetch_interval"` // how often to re-fetch during the day (e.g. "4h")
-	FetchWindowStart string  `yaml:"fetch_window_start"` // e.g. "07:00" — no fetches before this
-	FetchWindowEnd   string  `yaml:"fetch_window_end"`   // e.g. "19:00" — no fetches after this
+	Declination      int     `yaml:"declination"`        // panel tilt in degrees
+	Azimuth          int     `yaml:"azimuth"`            // degrees from south (south=0, west=90)
+	KWp              float64 `yaml:"kwp"`                // installed peak power
+	FetchInterval    string  `yaml:"fetch_interval"`     // how often to re-fetch during the day
+	FetchWindowStart string  `yaml:"fetch_window_start"` // e.g. "07:00"
+	FetchWindowEnd   string  `yaml:"fetch_window_end"`   // e.g. "19:00"
+	BaseLoadW        float64 `yaml:"base_load_w"`        // typical house consumption for surplus estimate (W)
+	MinSurplusW      float64 `yaml:"min_surplus_w"`      // min export needed to trigger wallbox charging (W)
 }
 
 func (f *ForecastConfig) FetchIntervalParsed() time.Duration {