fix: coerce a leading slash onto path bases

base: 'docs/' silently emitted relative-looking asset urls like
src="docs/assets/app.js".

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

__tests__/unit/node/config.test.ts

@@ -86,6 +86,15 @@ describe('node/config base normalization', () => {
       expect(normalizeSiteBase('/docs/')).toBe('/docs/')
     })
 
+    test('coerces a leading slash onto path bases', () => {
+      expect(normalizeSiteBase('docs')).toBe('/docs/')
+      expect(normalizeSiteBase('docs/')).toBe('/docs/')
+      expect(normalizeSiteBase('https://example.com/x')).toBe(
+        'https://example.com/x/'
+      )
+      expect(normalizeSiteBase('//cdn.example.com/')).toBe('//cdn.example.com/')
+    })
+
     test('normalizes relative forms to ./', () => {
       expect(normalizeSiteBase('.')).toBe('./')
       expect(normalizeSiteBase('./')).toBe('./')

src/node/config.ts

@@ -45,13 +45,20 @@ const resolve = (root: string, file: string) =>
   normalizePath(path.resolve(root, `.vitepress`, file))
 
 export function normalizeSiteBase(base?: string): string {
-  const normalized = base ? base.replace(/([^/])$/, '$1/') : '/'
+  let normalized = base ? base.replace(/([^/])$/, '$1/') : '/'
   if (normalized.startsWith('.') && !isRelativeBase(normalized)) {
     throw new Error(
       `a relative base must be exactly './' (got: ${base}) — pages always ` +
         `reference the site root relative to their own depth`
     )
   }
+  if (
+    !isRelativeBase(normalized) &&
+    !EXTERNAL_URL_RE.test(normalized) &&
+    !normalized.startsWith('/')
+  ) {
+    normalized = '/' + normalized
+  }
   return normalized
 }