fix(html): Windows 盘符路径解码去掉前导斜杠，修复 html 预览被 isWithin 拒绝

decodeHtmlUrl 对 'D:\' 盘符路径重拼为 '/D:/...'，node path.resolve() 在 Windows
上会变成 'D:\D:\...'（盘符重复），html 路由的 isWithin(cwd) 围栏因此拒绝所有盘
符路径——打开 html 报 "html path outside the session working directory"。

修复：首段匹配 /^[A-Za-z]:$/（盘符段）时改回无前导斜杠的 'D:/...' 形式，
requireAbsolute 可原样解析回盘符路径；POSIX 路径行为不变。

测试：html-route.spec 更新 Windows round-trip 断言，新增小写盘符用例与
resolve() 回归护栏（win32 平台守卫）。

tests/html-route.spec.ts

@@ -6,6 +6,7 @@
  * the session scope intact (the WHY of the path-encoding design).
  */
 import { describe, expect, it } from 'vitest'
+import { resolve } from 'node:path'
 import { decodeHtmlUrl, encodeHtmlUrl } from '../src/html-route.ts'
 
 describe('encodeHtmlUrl', () => {
@@ -38,11 +39,25 @@ describe('decodeHtmlUrl', () => {
     })
   })
 
-  it('decodes a Windows round-trip', () => {
+  it('decodes a Windows round-trip (drive path WITHOUT a leading slash, so node resolve() keeps the drive)', () => {
     const url = encodeHtmlUrl('sess-1', 'C:\\Users\\me\\a.html')
     expect(decodeHtmlUrl(url)).toEqual({
       ok: true,
-      ref: { sessionId: 'sess-1', path: '/C:/Users/me/a.html' },
+      ref: { sessionId: 'sess-1', path: 'C:/Users/me/a.html' },
+    })
+    // The host runs requireAbsolute() over the decoded path: with the old
+    // '/C:/...' form node's resolve() produced 'C:\C:\Users\...' (drive
+    // doubled) on Windows and the isWithin(cwd) fence rejected every drive
+    // path. The slash-free form must resolve back to the original path.
+    if (process.platform === 'win32') {
+      expect(resolve('C:/Users/me/a.html')).toBe('C:\\Users\\me\\a.html')
+    }
+  })
+
+  it('decodes a lowercase-drive Windows path without a leading slash', () => {
+    expect(decodeHtmlUrl('/sidebar/html/s/d%3A/work/x.html')).toEqual({
+      ok: true,
+      ref: { sessionId: 's', path: 'd:/work/x.html' },
     })
   })