feat(ui): elide overflowing text, tooltip full layer name

iced has no built-in ellipsis, so long user content (file names, layer
names, paths, property values) wrapped or spilled past fixed-width
panels. Add an `elide` helper (char-budget truncation with a trailing
ellipsis) and apply it to the layer manager, properties panel, document
tabs, recent-files list and the save dialog.

In the layer manager a truncated name reveals its full text in a
cursor-following tooltip on hover, so the column need not be widened.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/text_util.rs

@@ -0,0 +1,45 @@
+//! Shared helpers for fitting text into width-constrained widgets.
+//!
+//! iced has no built-in ellipsis, so long user content (file names, layer
+//! names, paths, property values) would otherwise wrap and break row
+//! heights or spill past fixed-width panels. `elide` trims to a character
+//! budget and marks the cut with a single ellipsis.
+
+/// Truncate `s` to at most `max` characters, replacing the overflow with a
+/// trailing ellipsis. `max` includes the ellipsis character; `max == 0`
+/// yields an empty string. Counts by `char` so multi-byte text is safe.
+pub fn elide(s: &str, max: usize) -> String {
+    if s.chars().count() <= max {
+        return s.to_string();
+    }
+    if max == 0 {
+        return String::new();
+    }
+    let mut out: String = s.chars().take(max - 1).collect();
+    out.push('…');
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::elide;
+
+    #[test]
+    fn short_unchanged() {
+        assert_eq!(elide("abc", 8), "abc");
+        assert_eq!(elide("abc", 3), "abc");
+    }
+
+    #[test]
+    fn long_truncated() {
+        assert_eq!(elide("abcdef", 4), "abc…");
+        assert_eq!(elide("abcdef", 1), "…");
+        assert_eq!(elide("abcdef", 0), "");
+    }
+
+    #[test]
+    fn multibyte_safe() {
+        // No panic on a char boundary and the budget counts chars.
+        assert_eq!(elide("çağrışım", 4), "çağ…");
+    }
+}