feat(FN-817): add expand button and swipe-friendly touch behavior for board cards

- Add expand button to TaskCard for opening task detail modal on mobile
- Implement CSS scroll-snap on board columns for swipe-friendly navigation
- Add touch-action and overscroll-behavior styles for smoother mobile scrolling
- Add comprehensive tests for mobile scroll snap behavior and expand button
- Document expand button and swipe-friendly board behavior in README

packages/dashboard/app/components/TaskCard.tsx

@@ -1,5 +1,5 @@
 import { memo, useCallback, useState, useRef, useEffect, useMemo } from "react";
-import { Link, Clock, Layers, Pencil, ChevronDown, Folder } from "lucide-react";
+import { Link, Clock, Layers, Pencil, ChevronDown, Folder, Maximize2 } from "lucide-react";
 import type { Task, TaskDetail, Column, PrInfo, IssueInfo } from "@fusion/core";
 import { fetchTaskDetail, uploadAttachment } from "../api";
 import { GitHubBadge } from "./GitHubBadge";
@@ -462,6 +462,11 @@ function TaskCardComponent({
     enterEditMode(e);
   }, [enterEditMode]);
 
+  const handleExpandClick = useCallback((e: React.MouseEvent<HTMLButtonElement>) => {
+    e.stopPropagation();
+    void handleClick();
+  }, [handleClick]);
+
   // Auto-resize textarea (similar to InlineCreateCard)
   const handleDescChange = useCallback((e: React.ChangeEvent<HTMLTextAreaElement>) => {
     setEditDescription(e.target.value);
@@ -575,6 +580,14 @@ function TaskCardComponent({
           />
         )}
         <div className="card-header-actions">
+          <button
+            className="card-expand-btn"
+            onClick={handleExpandClick}
+            title="Open task details"
+            aria-label="Open task details"
+          >
+            <Maximize2 size={12} />
+          </button>
           {canEdit && (
             <button
               className="card-edit-btn"