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>

frontend/src/components/WeatherCard.vue

@@ -7,6 +7,7 @@ interface ForecastDay {
   high: number
   low: number
   precip_probability: number | null
+  precip_mm: number | null
   windspeed_max: number
 }
 
@@ -90,7 +91,12 @@ const fetchedAtLabel = computed(() => {
         <span class="forecast-icon">{{ weatherIcon(day.condition) }}</span>
         <span class="forecast-condition">{{ day.condition }}</span>
         <span class="forecast-temps">{{ day.high }}° / {{ day.low }}°</span>
-        <span v-if="day.precip_probability != null && day.precip_probability > 0" class="forecast-precip">💧 {{ day.precip_probability }}%</span>
+        <span v-if="day.precip_probability != null && day.precip_probability > 0" class="forecast-precip">
+          💧 {{ day.precip_probability }}%
+        </span>
+        <span v-else-if="day.precip_mm != null && day.precip_mm > 0" class="forecast-precip">
+          💧 {{ day.precip_mm.toFixed(1) }}mm
+        </span>
         <span v-else class="forecast-precip forecast-precip--dry">💧 —</span>
         <span class="forecast-wind">💨 {{ day.windspeed_max }} {{ weather.wind_unit ?? 'km/h' }}</span>
       </div>