Weather animation polish: overcast sky, ground effects, matched dividers

[eetu]

Follow-up polish on the weather ASCII animation (#977).

Changes

Overcast sky rework

• Clouds and fog now paint a dim sky and draw lighter clouds on top, instead of grey-on-white (which read as smoke). The canvas paints the card surface itself, so only cloudy/foggy get the tint — other kinds keep the normal card.
• Partly cloudy (symbol 2) is split into its own kind: a bluer, sunnier sky with sparser clouds so the blue peeks through, distinct from full overcast (symbol 3).

Full-bleed animation through the footer

• Box gains an optional background slot rendered behind header + footer; those surfaces go transparent so the animation runs unbroken across the footer seam. Every other box omits the prop and is unchanged.

Hue-matched dividers

• The header/footer dividers and the morning/day/evening/night segment dividers now take a colour matched to the sky (a grey hairline next to a blue sky looked disconnected). Falls back to the default border for non-cloud weather.
• Extracted shared weatherSymbolToKind + new weatherBorderColor into weather/asciiSky.ts (single source of truth; the animation component imports the kind mapping instead of duplicating it).

Ground contact at the bottom edge

• Rain/thunder: drops flick a small splash on impact (bounded count).
• Snow: a per-column layer gathers along the bottom, slowly melts so it stays low and uneven, and each flake puffs a soft "thud" as it lands (landing on top of the existing pile).

Test plan

• yarn validate passes (typecheck, lint, format)
• ?demo → cycle puolipilv. / pilvinen / sumu to compare skies and dividers; rankka / lumi to see splashes and the gathering snow layer
• Confirm the animation is continuous across the footer and other boxes are visually unchanged