fix: resolve npm install peer dependency issues

Comprehensive fix for installer failures related to Astro/Starlight peer dependencies:

1. Update @astrojs/starlight from 0.37.0 to 0.37.5 (latest stable)
2. Add .npmrc with legacy-peer-deps to prevent peer dependency warnings
3. Update module installer to use --legacy-peer-deps flag for external modules

This resolves issues where:
- npm shows peer dependency warnings that users interpret as failures
- External module installations fail due to strict peer dependency checking
- Different npm versions handle peer dependencies inconsistently

Technical note: Starlight 0.37.x requires astro@^5.5.0, which is compatible
with astro@5.16.0 (^5.5.0 means >=5.5.0 <6.0.0). The issue was npm's warning
display, not actual incompatibility.

Fixes: Installation errors reported in v6.0.0-Beta.2

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.npmrc

@@ -1 +1,5 @@
-registry=https://registry.npmjs.org
+# Prevent peer dependency warnings during installation
+legacy-peer-deps=true
+
+# Improve install performance
+prefer-offline=true

package.json

@@ -88,7 +88,7 @@
   },
   "devDependencies": {
     "@astrojs/sitemap": "^3.6.0",
-    "@astrojs/starlight": "^0.37.0",
+    "@astrojs/starlight": "^0.37.5",
     "@eslint/js": "^9.33.0",
     "archiver": "^7.0.1",
     "astro": "^5.16.0",

tools/cli/installers/lib/modules/manager.js

@@ -416,7 +416,7 @@ class ModuleManager {
       if (needsDependencyInstall || wasNewClone || nodeModulesMissing) {
         const installSpinner = ora(`Installing dependencies for ${moduleInfo.name}...`).start();
         try {
-          execSync('npm install --production --no-audit --no-fund --prefer-offline --no-progress', {
+          execSync('npm install --production --no-audit --no-fund --prefer-offline --no-progress --legacy-peer-deps', {
             cwd: moduleCacheDir,
             stdio: 'pipe',
             timeout: 120_000, // 2 minute timeout
@@ -441,7 +441,7 @@ class ModuleManager {
         if (packageJsonNewer) {
           const installSpinner = ora(`Installing dependencies for ${moduleInfo.name}...`).start();
           try {
-            execSync('npm install --production --no-audit --no-fund --prefer-offline --no-progress', {
+            execSync('npm install --production --no-audit --no-fund --prefer-offline --no-progress --legacy-peer-deps', {
               cwd: moduleCacheDir,
               stdio: 'pipe',
               timeout: 120_000, // 2 minute timeout