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>

configs/ems-config.yaml

@@ -98,6 +98,8 @@ forecast:
   fetch_interval: "4h"      # re-fetch during the day (free tier: 12 req/day → 3 fetches)
   fetch_window_start: "07:00"
   fetch_window_end: "19:00"
+  base_load_w: 1000         # typical house consumption used for surplus estimate (W)
+  min_surplus_w: 1800       # min export surplus to trigger wallbox — matches wallbox_a threshold
 
 # EMS operational settings
 ems: