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-chat-ui.tsx

@@ -257,7 +257,7 @@ export function AgentPanelTabs<T extends string>({ value, items, theme, right, o
             <div className="flex min-h-9 items-center justify-between gap-2 rounded-md p-1" style={{ background: theme.spatial.surface }}>
                 <nav className="grid min-w-0 flex-1 grid-flow-col auto-cols-fr items-center gap-0.5 text-xs" role="tablist" aria-label="Agent 面板">
                     {items.map((item) => (
-                        <button key={item.value} type="button" role="tab" aria-selected={value === item.value} className={`inline-flex h-8 min-w-0 items-center justify-center gap-1 rounded-[5px] px-1.5 transition-colors ${value === item.value ? "font-medium" : "font-normal"}`} style={{ background: value === item.value ? theme.node.fill : "transparent", color: value === item.value ? theme.node.text : theme.node.muted, boxShadow: value === item.value ? `0 1px 5px ${theme.spatial.shadow}` : "none" }} onClick={() => onChange(item.value)}>
+                        <button key={item.value} type="button" role="tab" aria-selected={value === item.value} className={`inline-flex h-8 min-w-0 items-center justify-center gap-1 rounded-[var(--r-sm)] px-1.5 transition-colors ${value === item.value ? "font-medium" : "font-normal"}`} style={{ background: value === item.value ? theme.node.fill : "transparent", color: value === item.value ? theme.node.text : theme.node.muted, boxShadow: value === item.value ? `0 1px 5px ${theme.spatial.shadow}` : "none" }} onClick={() => onChange(item.value)}>
                             <span className="shrink-0">{item.icon}</span>
                             <span className="min-w-0 truncate">{item.label}</span>
                             {item.count ? <span className="shrink-0 tabular-nums opacity-60">{item.count}</span> : null}