Tab strip: shrink tabs before scrolling, keep Open in sight; ignore about:blank titles (#612)

* M3: tabs shrink to fit before they scroll, and Open stays in sight

With more documents open than fit, the tabs kept their full width: they ran
under the buttons on the right (#416 made them scroll) and the Open button
scrolled away with them. Now the tabs shrink first, as browsers do: narrow
tabs keep their width, wider ones share the rest equally and end their names
in "…" (the tooltip has the whole name). Only once they are at their narrowest
does the strip scroll. Open sits after the tabs, outside the scrolling area,
so it is always in sight next to the controls on the right.

Refs #397.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

* M3: a placeholder title like about:blank leaves the file name on the tab

A browser's "Save as PDF" of a blank page or pop-up titles the document
"about:blank" and asks viewers to show the title, so its tab and window said
"about:blank". A one-word browser address (about:, blob:, data:) or a bare
"Untitled" now counts as no title, and the file name is shown. A tab that
does show a document title names its file on hover.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

* M3: arrows step through tabs that don't fit, and tabs keep most of their name

Tabs shrank to 110 points before the strip scrolled, so with many open they
became a few characters each and nothing showed that more were hidden. Tabs
now stop at 180 points (most of a name), and once even that doesn't fit,
arrows appear either side of them: each steps about one tab, and an arrow is
dimmed at its end, so it is clear whether more tabs lie that way. The mouse
wheel still scrolls them, and the active tab is still brought into view.

New labels "Scroll tabs left" / "Scroll tabs right" are translated in every
catalog (machine-written; native review welcome).

Refs #397.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

---------

Co-authored-by: William Max & Co. <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>
Co-authored-by: Brandon Thomas <[email protected]>

crates/engine/src/tests.rs

@@ -2405,3 +2405,14 @@ fn comments_without_appearances_are_drawn_but_not_saved() {
         assert!(reopened.get(r).as_dict().is_some_and(|d| !d.contains(b"AP")), "{r:?} is saved without /AP");
     }
 }
+
+/// Titles a browser writes for a blank page or a pop-up name no document; real titles stay.
+#[test]
+fn placeholder_titles_are_recognised() {
+    for t in ["about:blank", "About:Blank", " about:srcdoc ", "blob:https://bank.example/3f2a", "data:text/html,x", "Untitled", "untitled"] {
+        assert!(is_placeholder_title(t), "{t:?}");
+    }
+    for t in ["Quarterly report", "Untitled report", "About: our company", "Statement", "blank"] {
+        assert!(!is_placeholder_title(t), "{t:?}");
+    }
+}