fix(KB-166): revert mobile board flex layout from KB-164

- Remove mobile board flex/snap-scroll CSS overrides from styles.css
- Remove Board tests asserting direct-child column structure and semantic tag
- Delete fix-mobile-board-double-scrollbar changeset
- Retain mobile modal full-screen overrides

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

@@ -52,23 +52,4 @@ describe("Board", () => {
       expect(screen.getByTestId(`column-${col}`)).toBeDefined();
     }
   });
-
-  it("renders all 5 columns as direct children of .board (CSS selector target)", () => {
-    renderBoard();
-    const board = screen.getByRole("main");
-    // The mock Column renders <div data-testid="column-{col}" />, which are direct children
-    const directChildren = Array.from(board.children);
-    expect(directChildren).toHaveLength(COLUMNS.length);
-    // Each direct child should be one of the column test-id elements
-    for (const col of COLUMNS) {
-      const colEl = screen.getByTestId(`column-${col}`);
-      expect(colEl.parentElement).toBe(board);
-    }
-  });
-
-  it("renders the board element as a <main> tag (semantic structure)", () => {
-    renderBoard();
-    const board = screen.getByRole("main");
-    expect(board.tagName).toBe("MAIN");
-  });
 });

packages/dashboard/app/styles.css

@@ -1296,44 +1296,10 @@ html, body {
   font-weight: 600;
 }
 
-/* === Mobile Responsive Overrides ===
-   On narrow viewports (≤768px) the board switches from a 5-column grid to a
-   horizontally-scrollable flex layout so only one scrollbar appears.  Each
-   column gets a fixed min-width and snap-scrolling for a polished swipe feel.
-   The modal also goes full-screen with reduced spacing. */
+/* === Mobile: Task Detail Modal ===
+   On narrow viewports (≤768px) the modal goes full-screen, action buttons wrap,
+   and spacing is reduced to stay usable on screens as small as 320px. */
 @media (max-width: 768px) {
-  /* Prevent ancestor elements from producing a second horizontal scrollbar */
-  html, body {
-    overflow: hidden;
-  }
-
-  #root {
-    overflow: hidden;
-  }
-
-  /* Board: flex layout with single horizontal scroll + snap */
-  .board {
-    display: flex;
-    overflow-x: auto;
-    overflow-y: hidden;
-    -webkit-overflow-scrolling: touch;
-    scroll-snap-type: x mandatory;
-    padding: 12px;
-    gap: 12px;
-  }
-
-  .board > .column {
-    min-width: 280px;
-    flex-shrink: 0;
-    scroll-snap-align: start;
-  }
-
-  /* Reduce header padding to reclaim horizontal space */
-  .header {
-    padding: 12px 12px;
-  }
-
-  /* Modal: full-screen on mobile */
   .modal-overlay {
     padding-top: 0;
     align-items: stretch;