fix: cap participant chat column to viewport on long forms

m: "The size of the chat should be limited to the screen size (- frame)
so it does not go via the whole long form..."

The grid layout (1fr 1fr at ≥900px) was stretching the chat column to
match the form column's height. With a 20-question form the right side
of the page became a 3000px scroll-soup of empty chat space.

Two-line fix:

- .fb-participant gets `align-items: start` at ≥900px — grid items take
  their own natural height instead of stretching to the tallest sibling.
- .fb-participant__col--chat gets `max-height: calc(100vh - 2rem)` always
  + `position: sticky; top: 1rem` at ≥900px. Long form scrolls past it;
  chat stays pinned in the viewport. On mobile (single-column stack) the
  existing 70vh cap stays — the desktop max-height applies on top of it,
  so mobile uses min(70vh, 100vh - 2rem) which is just 70vh.

No markup changes — pure CSS.

src/lib/styles/feedback.css

@@ -1666,6 +1666,10 @@ body { min-height: 100vh; }
 @media (min-width: 900px) {
 	.fb-participant {
 		grid-template-columns: 1fr 1fr;
+		/* Don't stretch the chat column to match a long form. Each column
+		   takes its own natural height; the chat column then caps itself
+		   to the viewport via .fb-participant__col--chat below. */
+		align-items: start;
 	}
 }
 
@@ -1700,12 +1704,24 @@ body { min-height: 100vh; }
 	}
 }
 
-/* Chat column — bubbles fill the column height, compose pinned at bottom. */
+/* Chat column — bubbles fill the column height, compose pinned at bottom.
+   The column always caps to the viewport so a long form on the left can't
+   stretch the chat into a 3000px tall scroll-soup. On desktop it sticks to
+   the top so it stays visible while the user scrolls down through the
+   form. On mobile it just caps below the form. */
 .fb-participant__col--chat {
 	background: var(--color-bg-primary);
 	border-radius: var(--radius-lg);
 	padding: var(--space-3);
 	min-height: 320px;
+	max-height: calc(100vh - 2rem);
+}
+
+@media (min-width: 900px) {
+	.fb-participant__col--chat {
+		position: sticky;
+		top: 1rem;
+	}
 }
 
 @media (max-width: 899px) {