fix: floating mini-chat overlay style + weather precip fallback

- KnowledgeView mini-chat: replace harsh border-top with upward box-shadow
  and rounded top corners (16px); remove padding-bottom from content area
  so widget truly overlays cards without pushing layout; add collapse
  toggle (chevron) that hides messages without closing the conversation
- WeatherCard: show precip_mm as fallback when precipitation_probability_max
  is null but actual rainfall is expected (Open-Meteo omits probability for
  some forecast days even when rain is shown)
- Pass precip_mm through weather service → frontend type definitions

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

src/fabledassistant/services/weather.py

@@ -155,6 +155,7 @@ def parse_weather_card_data(
                 "high": to_temp(d["temp_max"]),
                 "low": to_temp(d["temp_min"]),
                 "precip_probability": d["precip_probability"],
+                "precip_mm": d["precip_mm"],
                 "windspeed_max": to_wind(d["windspeed_max"]),
             }
             for d in future_days