feat(header): shrink long titles to fit instead of wrapping

The header keeps its size; a title that would overflow scales down to a
floor of 55% and ellipsizes only past that. Re-measures on title change,
resize, and web-font load.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0189ASmkMee4F5aJ3jnKeKQv

src/features/core/components/header.tsx

@@ -1,15 +1,57 @@
+import { useLayoutEffect, useRef } from "react";
 import { Title, AppShell, Flex } from "@mantine/core";
 import { useLingui } from "@lingui/react";
 import { HeaderConfig } from "../types/header-config";
 import BackButton from "./back-button";
 
+const MIN_TITLE_SCALE = 0.55;
+
 const Header = ({ collapsed, title, titleValues, withBackButton }: HeaderConfig) => {
   const { i18n } = useLingui();
+  const titleRef = useRef<HTMLHeadingElement>(null);
   const resolvedTitle =
     typeof title === "string" || title === undefined
       ? title
       : i18n._({ ...title, values: titleValues });
 
+  useLayoutEffect(() => {
+    const el = titleRef.current;
+    if (!el) return;
+    let cancelled = false;
+
+    const textWidth = () => {
+      const range = document.createRange();
+      range.selectNodeContents(el);
+      return range.getBoundingClientRect().width;
+    };
+
+    const fit = () => {
+      el.style.fontSize = "";
+      const available = el.clientWidth - 1;
+      if (available <= 0) return;
+      let needed = textWidth();
+      if (needed <= available) return;
+      const natural = parseFloat(getComputedStyle(el).fontSize);
+      let scale = Math.max(available / needed, MIN_TITLE_SCALE);
+      el.style.fontSize = `${natural * scale}px`;
+      for (let i = 0; i < 3 && scale > MIN_TITLE_SCALE && textWidth() > available; i++) {
+        scale = Math.max(scale * 0.98, MIN_TITLE_SCALE);
+        el.style.fontSize = `${natural * scale}px`;
+      }
+    };
+
+    fit();
+    document.fonts?.ready.then(() => {
+      if (!cancelled) fit();
+    });
+    const observer = new ResizeObserver(fit);
+    observer.observe(el);
+    return () => {
+      cancelled = true;
+      observer.disconnect();
+    };
+  }, [resolvedTitle]);
+
   return (
     <AppShell.Header
       id='app-header'
@@ -20,8 +62,20 @@ const Header = ({ collapsed, title, titleValues, withBackButton }: HeaderConfig)
       }}
     >
       { withBackButton && <BackButton /> }
-      <Flex justify='center' px='md' mt={8}>
+      <Flex justify='center' px={48} mt={8} w='100%'>
-        <Title order={1} lts='0.08em' style={{ userSelect: 'none' }}>
+        <Title
+          ref={titleRef}
+          order={1}
+          lts='0.08em'
+          style={{
+            userSelect: 'none',
+            width: '100%',
+            textAlign: 'center',
+            whiteSpace: 'nowrap',
+            overflow: 'hidden',
+            textOverflow: 'ellipsis',
+          }}
+        >
           {resolvedTitle?.toLocaleUpperCase()}
         </Title>
       </Flex>