Fix emoji variation selector width calculation in string_advance

Addresses issue where emoji with variation selectors (like ❣️, ♥️) were
incorrectly calculated as width 1 instead of width 2, causing display
overflow in modern terminals.

- Add variation selector detection (0xFE0F) in string_advance_with_grapheme_support()
- Apply special width handling for base emoji + variation selector combinations
- Ensures consistent width calculation with input field processing
- Fixes chat window overflow issues in terminals like Ghostty

This brings string_advance logic in line with unichar_array_advance_cluster
which already had proper variation selector handling.

src/core/utf8.c

@@ -42,6 +42,7 @@ static int string_advance_with_grapheme_support(char const **str, int policy)
 	const char *start = *str;
 	const char *pos = *str;
 	int cluster_width = 0;
+	int has_variation_selector = 0;
 	utf8proc_ssize_t bytes;
 
 	if (policy != TREAT_STRING_AS_UTF8) {
@@ -69,6 +70,11 @@ static int string_advance_with_grapheme_support(char const **str, int policy)
 			break;
 		}
 
+		/* Check for variation selector */
+		if (codepoint == 0xFE0F) {
+			has_variation_selector = 1;
+		}
+
 		/* Add this codepoint's width to the cluster */
 		if (unichar_isprint(codepoint)) {
 			int char_width = i_wcwidth(codepoint);
@@ -81,6 +87,12 @@ static int string_advance_with_grapheme_support(char const **str, int policy)
 		pos += bytes;
 	}
 
+	/* Special handling for emoji with variation selector */
+	if (has_variation_selector && cluster_width == 1) {
+		/* Base emoji (like ❣ U+2763, ♥ U+2665) + variation selector should have width 2 */
+		cluster_width = 2;
+	}
+
 	*str = pos;
 	return cluster_width > 0 ? cluster_width : 1;
 }