fix(scripts): live mermaid playwright wrapper runs from repo root

HAPI Bot Minor (PR tiann#741): the wrapper sets cwd to scripts/, but the
test:mermaid-lightbox:live npm script lives in the repo-root
package.json, so spawning npm there exited before Playwright started.
Switch cwd to the repo root and drop the unused WEB_DIR constant.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: heavygee

scripts/dev/mermaid-lightbox-live-playwright.mjs

@@ -4,14 +4,14 @@ import { spawnSync } from 'node:child_process'
 import { dirname, resolve } from 'node:path'
 import { fileURLToPath } from 'node:url'
 
-const WEB_DIR = resolve(dirname(fileURLToPath(import.meta.url)), '../../web')
+const REPO_ROOT = resolve(dirname(fileURLToPath(import.meta.url)), '../..')
 const npmBin = process.env.NPM_BIN ?? 'npm'
 
 const result = spawnSync(
     npmBin,
     ['run', 'test:mermaid-lightbox:live'],
     {
-        cwd: resolve(dirname(fileURLToPath(import.meta.url)), '..'),
+        cwd: REPO_ROOT,
         stdio: 'inherit',
         env: { ...process.env, PATH: process.env.PATH, HAPI_LIVE: '1' },
     },