feat(ui): P4 无障碍与打磨收尾

- P4-1 对比度：muted-soft 提到 WCAG AA 小字(≥4.5:1)于两主题各面
- P4-3 ConflictCard 原文/建议加 Minus/Plus 图标 + sr-only 标签（不靠颜色）
- P4-4 动效：3 处 chevron transition-transform 加 motion-safe 门控
- P4-5 命令面板 ⌘K 键帽提示 + 键盘操作读数 + 空态引导 + mono 快捷键
- P2-8 SegmentedControl 升级 radiogroup（role=radio/aria-checked + 方向键 roving）

apps/web/components/ui/SegmentedControl.tsx

@@ -1,4 +1,5 @@
-import type { ReactNode } from "react";
+import type { KeyboardEvent, ReactNode } from "react";
+import { useRef } from "react";
 
 import { cn, focusRing, segmentedClass, transitionUi } from "@/lib/ui/variants";
 
@@ -22,16 +23,57 @@ export function SegmentedControl<T extends string>({
   ariaLabel,
   className,
 }: SegmentedControlProps<T>) {
+  const buttonsRef = useRef<Array<HTMLButtonElement | null>>([]);
+
+  const focusOption = (index: number) => {
+    const clamped = (index + options.length) % options.length;
+    const target = options[clamped];
+    if (!target) return;
+    onChange(target.value);
+    buttonsRef.current[clamped]?.focus();
+  };
+
+  const handleKeyDown = (event: KeyboardEvent<HTMLButtonElement>, index: number) => {
+    switch (event.key) {
+      case "ArrowRight":
+      case "ArrowDown":
+        event.preventDefault();
+        focusOption(index + 1);
+        break;
+      case "ArrowLeft":
+      case "ArrowUp":
+        event.preventDefault();
+        focusOption(index - 1);
+        break;
+      case "Home":
+        event.preventDefault();
+        focusOption(0);
+        break;
+      case "End":
+        event.preventDefault();
+        focusOption(options.length - 1);
+        break;
+      default:
+        break;
+    }
+  };
+
   return (
-    <div className={segmentedClass(className)} role="group" aria-label={ariaLabel}>
-      {options.map((option) => {
+    <div className={segmentedClass(className)} role="radiogroup" aria-label={ariaLabel}>
+      {options.map((option, index) => {
         const selected = option.value === value;
         return (
           <button
             key={option.value}
+            ref={(node) => {
+              buttonsRef.current[index] = node;
+            }}
             type="button"
-            aria-pressed={selected}
+            role="radio"
+            aria-checked={selected}
+            tabIndex={selected ? 0 : -1}
             onClick={() => onChange(option.value)}
+            onKeyDown={(event) => handleKeyDown(event, index)}
             className={cn(
               "rounded-md px-3 py-1.5 text-sm",
               transitionUi,