feat(weather): overcast sky rework, ground effects, matched dividers

- Clouds/fog: paint a dim sky and draw lighter clouds (was grey-on-white
  smoke). Split partly-cloudy (symbol 2) into its own kind with a bluer,
  sunnier sky and sparser clouds so blue peeks through.
- Box gains a full-bleed background slot: header/footer go transparent so
  the animation runs unbroken through the footer. Other boxes unchanged.
- Dividers (header/footer + the morning/day/evening/night segments) take a
  hue matched to the sky (shared weatherSymbolToKind + weatherBorderColor in
  weather/asciiSky.ts).
- Ground contact at the bottom edge: rain splashes, a melting snow layer
  that gathers per-column, and soft thud puffs as flakes land.

Author: eetu

frontend/src/components/Box.tsx

@@ -8,9 +8,21 @@ type BoxProps = {
   className?: string;
   drawer?: React.ReactElement;
   loading?: boolean;
+  // full-bleed layer rendered behind header + footer; when set, those surfaces
+  // turn transparent so it shows through continuously (e.g. weather animation)
+  background?: React.ReactElement;
+  // overrides the header/footer divider colour (e.g. to match a sky backdrop)
+  borderColor?: string;
 } & React.HTMLAttributes<HTMLDivElement>;
 
-const Box: React.FC<BoxProps> = ({ className, children, drawer, loading }) => {
+const Box: React.FC<BoxProps> = ({
+  className,
+  children,
+  drawer,
+  loading,
+  background,
+  borderColor,
+}) => {
   const [collapsed, setCollapsed] = useState(true);
   const theme = useTheme();
 
@@ -35,6 +47,7 @@ const Box: React.FC<BoxProps> = ({ className, children, drawer, loading }) => {
     <div
       className={className}
       css={{
+        position: "relative",
         cursor: expandable ? "pointer" : "default",
         minWidth: 0,
         overflow: "hidden",
@@ -43,11 +56,26 @@ const Box: React.FC<BoxProps> = ({ className, children, drawer, loading }) => {
       }}
       onClick={expandable ? () => setCollapsed(!collapsed) : undefined}
     >
-      <BoxHeader hairline={expandable}>{children}</BoxHeader>
+      {background && (
+        <div css={{ position: "absolute", inset: 0, zIndex: 0 }}>
+          {background}
+        </div>
+      )}
+      <BoxHeader
+        hairline={expandable}
+        transparent={background !== undefined}
+        borderColor={borderColor}
+      >
+        {children}
+      </BoxHeader>
       {expandable && (
         <>
           <BoxDrawer collapsed={collapsed}>{drawer}</BoxDrawer>
-          <BoxFooter collapsed={collapsed} />
+          <BoxFooter
+            collapsed={collapsed}
+            transparent={background !== undefined}
+            borderColor={borderColor}
+          />
         </>
       )}
     </div>
@@ -58,21 +86,33 @@ export default Box;
 
 type BoxHeaderProps = {
   hairline: boolean;
+  transparent?: boolean;
+  borderColor?: string;
 } & React.HTMLAttributes<HTMLDivElement>;
 
-const BoxHeader: React.FC<BoxHeaderProps> = ({ children, hairline }) => {
+const BoxHeader: React.FC<BoxHeaderProps> = ({
+  children,
+  hairline,
+  transparent,
+  borderColor,
+}) => {
   const theme = useTheme();
 
   return (
     <div
       css={{
         position: "relative",
+        zIndex: 1,
         overflow: "hidden",
         display: "flex",
         flexDirection: "column",
-        backgroundColor: theme.colors.background.main,
+        backgroundColor: transparent
+          ? "transparent"
+          : theme.colors.background.main,
         padding: "1.5em",
-        borderBottom: hairline ? `1px ${theme.colors.border} solid` : "none",
+        borderBottom: hairline
+          ? `1px ${borderColor ?? theme.colors.border} solid`
+          : "none",
       }}
     >
       {children}
@@ -92,6 +132,8 @@ const BoxDrawer: React.FC<BoxDrawerProps> = ({ children, collapsed }) => {
     <div
       ref={wrapperRef}
       css={{
+        position: "relative",
+        zIndex: 1,
         display: "grid",
         gridTemplateRows: collapsed ? "0fr" : "1fr",
         transition: "grid-template-rows 0.15s ease",
@@ -118,19 +160,31 @@ const BoxDrawer: React.FC<BoxDrawerProps> = ({ children, collapsed }) => {
 
 type BoxFooterProps = {
   collapsed: boolean;
+  transparent?: boolean;
+  borderColor?: string;
 } & React.HTMLAttributes<HTMLDivElement>;
 
-const BoxFooter: React.FC<BoxFooterProps> = ({ collapsed }) => {
+const BoxFooter: React.FC<BoxFooterProps> = ({
+  collapsed,
+  transparent,
+  borderColor,
+}) => {
   const theme = useTheme();
 
   return (
     <div
       css={{
+        position: "relative",
+        zIndex: 1,
         display: "flex",
         justifyContent: "center",
         height: "25px",
-        backgroundColor: theme.colors.background.main,
-        borderTop: collapsed ? "none" : `1px ${theme.colors.border} solid`,
+        backgroundColor: transparent
+          ? "transparent"
+          : theme.colors.background.main,
+        borderTop: collapsed
+          ? "none"
+          : `1px ${borderColor ?? theme.colors.border} solid`,
         color: theme.colors.text.light,
       }}
     >

frontend/src/components/FmiWeatherBox.tsx

@@ -10,6 +10,7 @@ import useLocationSettings from "../hooks/useLocationSettings";
 import useScreenshotMode from "../hooks/useScreenshotMode";
 import { PvForecast } from "../types/pv";
 import { WeatherData } from "../types/weather/fmi";
+import { weatherBorderColor } from "../weather/asciiSky";
 import {
   getFmiWeatherDescription,
   getFmiWeatherIcon,
@@ -159,11 +160,24 @@ const WeatherBox: React.FC<WeatherBoxProps> = ({ className }) => {
       ? testCase.precip
       : current.precipitation1h;
   const title = getFmiWeatherDescription(weatherSymbol);
+  const borderColor = weatherBorderColor(
+    weatherSymbol,
+    isNight,
+    theme.mode === "dark",
+  );
 
   return (
     <Box
       loading={!data}
       className={className}
+      borderColor={borderColor}
+      background={
+        <WeatherAsciiBackground
+          weatherSymbol={weatherSymbol}
+          isNight={isNight}
+          precipitation={precipitation}
+        />
+      }
       drawer={
         <div
           css={{
@@ -177,11 +191,6 @@ const WeatherBox: React.FC<WeatherBoxProps> = ({ className }) => {
         </div>
       }
     >
-      <WeatherAsciiBackground
-        weatherSymbol={weatherSymbol}
-        isNight={isNight}
-        precipitation={precipitation}
-      />
       {demo && (
         <WeatherTestControls
           activeId={testCase?.id ?? "live"}
@@ -326,7 +335,7 @@ const WeatherBox: React.FC<WeatherBoxProps> = ({ className }) => {
             css={{
               display: "flex",
               flexDirection: "column",
-              borderRight: `1px ${theme.colors.border} solid`,
+              borderRight: `1px ${borderColor ?? theme.colors.border} solid`,
               width: "25%",
               textAlign: "center",
               "&:last-of-type": {