refactor(canvas): 硬编码字号/圆角清理 → 语义 token（审计 D1/P1-3） (#131)

清理画布组件散落的硬编码字号与圆角，统一走 token 体系：

字号：
- 新增 --fs-nano(7px) 覆盖原 text-[6px]/text-[7px] 子微型标注
  （角标计数、节点预览、% 后缀）
- text-[19px] → --fs-heading-lg(18px)

圆角（就近映射，±1~2px）：
- rounded-[3px] → --r-xs | rounded-[5px] → --r-sm
- rounded-[6px] → --r-sm | rounded-[7px] → --r-md
- rounded-[18px] → --r-2xl

涉及 11 个组件文件。rounded-[1px]（minimap 视口指示框，刻意尖角）保留。
颜色项（bg-white/border-stone）经核实均带 dark: 变体，属标准 Tailwind
双主题模式非误用，不在本轮处理。

Co-authored-by: sikongyue <3155157671@qq.com>

web/src/components/canvas/canvas-agent-panel-chrome.tsx

@@ -73,7 +73,7 @@ function AgentModeSwitch({ value, theme, onChange }: { value: CanvasAgentMode; t
                 const active = value === item;
                 const Icon = item === "online" ? Globe2 : Laptop;
                 return (
-                    <button key={item} type="button" className="inline-flex h-7 items-center gap-1 rounded-[5px] px-2 transition-colors" style={{ background: active ? theme.node.fill : "transparent", color: active ? theme.node.text : theme.node.muted, boxShadow: active ? `0 1px 5px ${theme.spatial.shadow}` : "none" }} onClick={() => onChange(item)} aria-pressed={active}>
+                    <button key={item} type="button" className="inline-flex h-7 items-center gap-1 rounded-[var(--r-sm)] px-2 transition-colors" style={{ background: active ? theme.node.fill : "transparent", color: active ? theme.node.text : theme.node.muted, boxShadow: active ? `0 1px 5px ${theme.spatial.shadow}` : "none" }} onClick={() => onChange(item)} aria-pressed={active}>
                         <Icon className="size-3" />
                         {item === "online" ? "网站" : "本机"}
                     </button>
@@ -100,7 +100,7 @@ export function AgentChatEmptyState({ theme, nodeCount, onSelect }: { theme: Can
                     <span className="grid size-7 place-items-center rounded-md" style={{ background: theme.accent.primarySoft, color: theme.accent.primary }}><Bot className="size-3.5" /></span>
                     <span className="text-[var(--fs-label)] font-medium" style={{ color: theme.node.muted }}>{nodeCount} 个节点已就绪</span>
                 </div>
-                <h2 className="mt-3 text-[19px] font-semibold leading-6" style={{ color: theme.node.text }}>从当前画布开始</h2>
+                <h2 className="mt-3 text-[var(--fs-heading-lg)] font-semibold leading-6" style={{ color: theme.node.text }}>从当前画布开始</h2>
                 <div className="mt-4 grid grid-cols-1 gap-1">
                     {visibleStarterActions.map(({ label, icon: Icon }) => (
                         <button key={label} type="button" className="group flex min-h-11 min-w-0 items-center gap-2.5 rounded-md px-2.5 text-left text-xs font-medium transition-colors" style={{ color: theme.node.text }} onMouseEnter={(event) => { event.currentTarget.style.background = theme.spatial.surface; }} onMouseLeave={(event) => { event.currentTarget.style.background = "transparent"; }} onFocus={(event) => { event.currentTarget.style.background = theme.spatial.surface; }} onBlur={(event) => { event.currentTarget.style.background = "transparent"; }} onClick={() => onSelect(label)}>