feat(tabs): polish new-chat button — stuck-only square edge, New Chat labelHEADmain

- isStuckToEnd (pure): square the sticky '+' right edge only while it floats
  over scrolled tabs; rounded at rest. Edge-measured in TabBar via a disposed
  scroll + ResizeObserver effect (RO guarded for non-browser envs).
- Show a temporary 'New Chat' title when the draft is selected, with the '+'
  moved to the trailing close-button slot for consistency with real tabs.

1 files changed, 30 insertions, 0 deletions

diff --git a/src/features/tabs/ui.test.ts b/src/features/tabs/ui.test.ts
index 53df0be..1ae18c8 100644
--- a/src/features/tabs/ui.test.ts
+++ b/src/features/tabs/ui.test.ts
@@ -145,4 +145,34 @@ describe("TabBar", () => {
 		const newChat = screen.getByRole("button", { name: "New chat" });
 		expect(newChat).toHaveClass("sticky");
 	});
+
+	it("shows visible 'New Chat' text when activeConversationId is null", () => {
+		render(TabBar, {
+			props: {
+				tabs: sampleTabs,
+				activeConversationId: null,
+				onSelect: vi.fn(),
+				onClose: vi.fn(),
+				onNewDraft: vi.fn(),
+			},
+		});
+
+		const newChat = screen.getByRole("button", { name: "New chat" });
+		expect(newChat).toHaveTextContent("New Chat");
+	});
+
+	it("does not show 'New Chat' text when a real tab is active", () => {
+		render(TabBar, {
+			props: {
+				tabs: sampleTabs,
+				activeConversationId: "c1",
+				onSelect: vi.fn(),
+				onClose: vi.fn(),
+				onNewDraft: vi.fn(),
+			},
+		});
+
+		const newChat = screen.getByRole("button", { name: "New chat" });
+		expect(newChat).not.toHaveTextContent("New Chat");
+	});
 });