Stop the drag from selecting the tab's text

A pointer-driven drag is still a mouse drag as far as the browser is
concerned, so moving a tab highlighted its label blue — something the OS
drag image never did, and the last visible difference between this and
a real drag.

`select-none` on the tab. The rename field gets `select-text` back:
`user-select` inherits, and selecting text is exactly what that field is
for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/components/layout/MainTabs.test.tsx

@@ -127,6 +127,19 @@ describe("MainTabs reordering", () => {
     expect(useAppState.getState().activeTabKey).toBe(HOME);
   });
 
+  it("does not let a drag select the tab's text", () => {
+    // A pointer-driven drag is still a mouse drag as far as the browser is
+    // concerned, so without this the label highlights blue while you move it.
+    // The rename field is exempt — selecting there is the whole point.
+    render(<MainTabs />);
+    for (const tab of screen.getAllByRole("tab")) {
+      expect(tab.className).toContain("select-none");
+    }
+
+    fireEvent.doubleClick(screen.getAllByRole("tab")[1]);
+    expect(screen.getByLabelText("Rename tab").className).toContain("select-text");
+  });
+
   it("shows the tab itself under the cursor while dragging", () => {
     // A dimmed source tab and a thin line do not read as "I am holding this
     // tab" — the dragged copy is what makes the gesture legible.