feat(FN-3133): restore mobile back button with CSS contract tests

This merge adds a standalone bottom-gap CSS token for PWA mobile compatibility, implements a responsive back button that hides on desktop and restores on mobile, refactors the test-changed script for improved workflow testing, and expands the test suite with new CI workflow tests and mobile CSS cont

Fusion-Task-Id: FN-3133

packages/dashboard/app/components/MissionManager.css

@@ -125,6 +125,12 @@
   outline: none;
 }
 
+/* Hide back button on desktop — sidebar is always visible so navigation is unnecessary.
+   Keep control mounted in DOM for jsdom test compatibility; mobile override restores visibility. */
+.mission-manager--desktop .mission-manager__back-btn {
+  display: none;
+}
+
 /* ── Body ── */
 .mission-manager__body {
   flex: 1;
@@ -2197,6 +2203,11 @@
     padding: var(--space-md);
   }
 
+  /* Back button visible on mobile — stacked list→detail navigation needs it. */
+  .mission-manager--desktop .mission-manager__back-btn {
+    display: inline-flex;
+  }
+
   /* Restore dynamic header title text on mobile. */
   .mission-manager__title-text--mobile {
     display: inline;

packages/dashboard/app/components/MissionManager.tsx

@@ -3727,7 +3727,7 @@ export function MissionManager({ isOpen, isInline = false, onClose, addToast, pr
   const manager = (
     <div
       ref={modalRef}
-      className={`mission-manager mission-manager--desktop-split${isInline ? " mission-manager--inline" : ""}`}
+      className={`mission-manager mission-manager--desktop${isInline ? " mission-manager--inline" : ""}`}
       role={isInline ? undefined : "dialog"}
       aria-modal={isInline ? undefined : true}
       aria-label={isInline ? undefined : "Mission Manager"}
@@ -3735,7 +3735,7 @@ export function MissionManager({ isOpen, isInline = false, onClose, addToast, pr
     >
       <div className={`mission-manager__header${isInline ? " mission-manager__header--inline" : ""}`}>
         <div className="mission-manager__header-title">
-          {isMobile && selectedMission && (
+          {selectedMission && (
             <button
               className="mission-manager__back-btn"
               onClick={handleBackToList}

packages/dashboard/app/components/__tests__/MissionManager.mobile-css.test.ts

@@ -58,6 +58,16 @@ describe("MissionManager mobile styles", () => {
     expect(section).toContain("display: block;");
   });
 
+  it("hides back button on desktop and restores it on mobile", () => {
+    const css = loadAllAppCss();
+    const desktopRule = css.match(/\.mission-manager--desktop \.mission-manager__back-btn\s*\{[^}]*\}/)?.[0];
+    expect(desktopRule).toContain("display: none;");
+
+    const section = getMissionMobileSection(css);
+    const mobileRule = section.match(/\.mission-manager--desktop \.mission-manager__back-btn\s*\{[^}]*\}/)?.[0];
+    expect(mobileRule).toContain("display: inline-flex;");
+  });
+
   it("keeps desktop defaults and mobile overrides for header title spans", () => {
     const css = loadAllAppCss();
     const desktopMobileSpanBlock = css.match(/\.mission-manager__title-text--mobile\s*\{[^}]*\}/)?.[0];

packages/dashboard/app/components/__tests__/MissionManager.test.tsx

@@ -1416,8 +1416,8 @@ describe("MissionManager", () => {
 
     // Wait for detail view to render
     await waitFor(() => {
-      // Desktop keeps sidebar visible and does not render the mobile back button.
-      expect(screen.queryByTestId("mission-back-btn")).toBeNull();
+      // Desktop keeps sidebar visible and back button stays mounted (CSS-hidden).
+      expect(screen.getByTestId("mission-back-btn")).toBeInTheDocument();
       // Milestone should be visible (auto-expanded)
       expect(screen.getByText("Database Schema")).toBeDefined();
     });
@@ -1434,7 +1434,7 @@ describe("MissionManager", () => {
     fireEvent.click(screen.getByText("Build Auth System"));
 
     await waitFor(() => {
-      expect(screen.queryByTestId("mission-back-btn")).toBeNull();
+      expect(screen.getByTestId("mission-back-btn")).toBeInTheDocument();
       expect(screen.getByText("API Redesign")).toBeDefined();
     });
   });
@@ -1461,7 +1461,7 @@ describe("MissionManager", () => {
     });
   });
 
-  it("does not render back button on desktop in detail view", async () => {
+  it("keeps back button mounted on desktop in detail view", async () => {
     globalThis.fetch = createDetailFetchMock();
     render(<MissionManager isOpen={true} onClose={vi.fn()} addToast={vi.fn()} />);
 
@@ -1472,7 +1472,7 @@ describe("MissionManager", () => {
     fireEvent.click(screen.getByText("Build Auth System"));
 
     await waitFor(() => {
-      expect(screen.queryByLabelText("Back to missions list")).toBeNull();
+      expect(screen.getByLabelText("Back to missions list")).toBeInTheDocument();
     });
   });
 
@@ -3180,6 +3180,49 @@ describe("MissionManager", () => {
     });
   });
 
+  describe("desktop back button behavior", () => {
+    it("back button element exists when mission is selected and root uses desktop shell class", async () => {
+      mockViewport("desktop");
+      globalThis.fetch = createDetailFetchMock();
+      render(<MissionManager isOpen={true} onClose={vi.fn()} addToast={vi.fn()} />);
+
+      await waitFor(() => expect(screen.getByText("Build Auth System")).toBeInTheDocument());
+      fireEvent.click(screen.getByText("Build Auth System"));
+
+      await waitFor(() => {
+        expect(screen.getByTestId("mission-back-btn")).toBeInTheDocument();
+      });
+
+      expect(screen.getByTestId("mission-manager-dialog")).toHaveClass("mission-manager--desktop");
+    });
+
+    it("clicking back button clears selected mission", async () => {
+      mockViewport("desktop");
+      globalThis.fetch = createDetailFetchMock();
+      render(<MissionManager isOpen={true} onClose={vi.fn()} addToast={vi.fn()} />);
+
+      await waitFor(() => expect(screen.getByText("Build Auth System")).toBeInTheDocument());
+      fireEvent.click(screen.getByText("Build Auth System"));
+
+      await waitFor(() => expect(document.querySelector(".mission-manager__detail-pane .mission-detail")).toBeTruthy());
+      fireEvent.click(screen.getByTestId("mission-back-btn"));
+
+      await waitFor(() => {
+        expect(document.querySelector(".mission-manager__detail-pane .mission-detail")).toBeNull();
+        expect(screen.getByText("API Redesign")).toBeInTheDocument();
+      });
+    });
+
+    it("back button does not render when no mission is selected", async () => {
+      mockViewport("desktop");
+      globalThis.fetch = createDetailFetchMock();
+      render(<MissionManager isOpen={true} onClose={vi.fn()} addToast={vi.fn()} />);
+
+      await waitFor(() => expect(screen.getByText("Build Auth System")).toBeInTheDocument());
+      expect(screen.queryByTestId("mission-back-btn")).toBeNull();
+    });
+  });
+
   describe("mobile stacked layout", () => {
     it("renders stacked body on mobile and hides desktop split", async () => {
       mockViewport("mobile");