feat(FN-788): nest Scripts under Terminal in compact overflow menu

- Restructure Header.tsx to render Scripts as a submenu of Terminal in compact/tablet overflow menus
- Add submenu toggle state, keyboard navigation, and click-outside-to-close behavior
- Add CSS for nested submenu styling with slide-in animation and depth indicators
- Update Header tests to verify Scripts nested under Terminal with toggle interaction
- Update mobile-header-controls and tablet-header-controls tests for new menu hierarchy
- Update README documentation to reflect the Terminal > Scripts submenu structure

packages/dashboard/app/__tests__/mobile-header-controls.test.ts

@@ -72,6 +72,13 @@ describe("mobile-header-controls.css", () => {
     expect(cssContent).toMatch(/\.mobile-overflow-item:hover/);
   });
 
+  it("has terminal submenu styles for nested scripts under terminal", () => {
+    expect(cssContent).toContain(".mobile-overflow-group");
+    expect(cssContent).toContain(".mobile-overflow-submenu");
+    expect(cssContent).toContain(".mobile-overflow-subitem");
+    expect(cssContent).toContain(".mobile-overflow-chevron");
+  });
+
   it("does not contain obsolete mobile header search wrap rules", () => {
     // The old @media (max-width: 640px) and @media (max-width: 480px) 
     // header search rules should be removed

packages/dashboard/app/__tests__/tablet-header-controls.test.tsx

@@ -172,10 +172,18 @@ describe("tablet header controls", () => {
     expect(screen.getByText("Import from GitHub")).toBeDefined();
   });
 
-  it("overflow menu contains terminal on tablet", () => {
+  it("overflow menu contains terminal group on tablet", () => {
     renderTabletHeader({ onToggleTerminal: noop });
     fireEvent.click(screen.getByTitle("More header actions"));
+    expect(screen.getByTestId("overflow-terminal-group-trigger")).toBeDefined();
+  });
+
+  it("overflow menu contains terminal submenu items when expanded on tablet", () => {
+    renderTabletHeader({ onToggleTerminal: noop, onOpenScripts: noop });
+    fireEvent.click(screen.getByTitle("More header actions"));
+    fireEvent.click(screen.getByTestId("overflow-terminal-group-trigger"));
     expect(screen.getByTestId("overflow-terminal-btn")).toBeDefined();
+    expect(screen.getByTestId("overflow-scripts-btn")).toBeDefined();
   });
 
   it("overflow menu contains scheduled tasks on tablet", () => {
@@ -230,10 +238,11 @@ describe("tablet header controls", () => {
     expect(onOpenSettings).toHaveBeenCalled();
   });
 
-  it("calls onToggleTerminal from overflow menu on tablet", () => {
+  it("calls onToggleTerminal from terminal submenu on tablet", () => {
     const onToggleTerminal = vi.fn();
     renderTabletHeader({ onToggleTerminal });
     fireEvent.click(screen.getByTitle("More header actions"));
+    fireEvent.click(screen.getByTestId("overflow-terminal-group-trigger"));
     fireEvent.click(screen.getByTestId("overflow-terminal-btn"));
     expect(onToggleTerminal).toHaveBeenCalled();
   });
@@ -278,6 +287,17 @@ describe("tablet header controls", () => {
     expect(screen.queryByRole("menu")).toBeNull();
   });
 
+  it("closes terminal submenu on Escape without closing overflow menu on tablet", () => {
+    renderTabletHeader({ onToggleTerminal: noop });
+    fireEvent.click(screen.getByTitle("More header actions"));
+    fireEvent.click(screen.getByTestId("overflow-terminal-group-trigger"));
+    expect(screen.getByTestId("overflow-terminal-btn")).toBeDefined();
+    fireEvent.keyDown(document, { key: "Escape" });
+    // Submenu closes but overflow menu stays open
+    expect(screen.queryByTestId("overflow-terminal-btn")).toBeNull();
+    expect(screen.getByRole("menu")).toBeDefined();
+  });
+
   // ── Search on tablet ───────────────────────────────────────────
 
   it("renders desktop-style search input on tablet (not mobile search trigger)", () => {