feat: add browser environment compatibility support

- Remove top-level Node.js imports that break bundlers
- Use universal adapters for crypto operations
- Add dynamic imports for Node.js-specific modules
- Add browser field to package.json for bundler hints
- Maintain full Node.js functionality while enabling browser usage

This allows Brainy to be used with modern bundlers (Vite, Webpack, etc.)
without requiring Node.js polyfills. Browser environments get core features
while Node.js retains all capabilities including filesystem and networking.

src/critical/model-guardian.ts

@@ -11,11 +11,8 @@
  * 4. System MUST fail fast if model unavailable in production
  */
 
-import { existsSync } from 'node:fs'
-import { readFile, mkdir, writeFile, stat } from 'node:fs/promises'
-import { join, dirname } from 'node:path'
-import { createHash } from 'node:crypto'
 import { env } from '@huggingface/transformers'
+import { createHash } from '../universal/crypto.js'
 
 // CRITICAL: These values MUST NEVER CHANGE
 const CRITICAL_MODEL_CONFIG = {
@@ -102,8 +99,8 @@ export class ModelGuardian {
       return
     }
     
-    // Step 2: In production, FAIL FAST
-    if (process.env.NODE_ENV === 'production' && !process.env.BRAINY_ALLOW_RUNTIME_DOWNLOAD) {
+    // Step 2: In production, FAIL FAST (Node.js only)
+    if (typeof window === 'undefined' && process.env.NODE_ENV === 'production' && !process.env.BRAINY_ALLOW_RUNTIME_DOWNLOAD) {
       throw new Error(
         '🚨 CRITICAL FAILURE: Transformer model not found in production!\n' +
         'The model is REQUIRED for Brainy to function.\n' +
@@ -147,7 +144,18 @@ export class ModelGuardian {
    * Verify the local model files exist and are correct
    */
   private async verifyLocalModel(): Promise<boolean> {
-    const modelBasePath = join(this.modelPath, ...CRITICAL_MODEL_CONFIG.modelName.split('/'))
+    // Browser doesn't have local file access
+    if (typeof window !== 'undefined') {
+      console.log('⚠️ Model verification skipped in browser environment')
+      return false
+    }
+
+    // Dynamically import Node.js modules
+    const fs = await import('node:fs')
+    const fsPromises = await import('node:fs/promises')
+    const path = await import('node:path')
+
+    const modelBasePath = path.join(this.modelPath, ...CRITICAL_MODEL_CONFIG.modelName.split('/'))
     console.log(`🔍 Debug: Checking model at path: ${modelBasePath}`)
     console.log(`🔍 Debug: Model path components: ${this.modelPath} + ${CRITICAL_MODEL_CONFIG.modelName.split('/')}`)
     
@@ -159,17 +167,17 @@ export class ModelGuardian {
     ]
     
     for (const file of criticalFiles) {
-      const filePath = join(modelBasePath, file)
+      const filePath = path.join(modelBasePath, file)
       console.log(`🔍 Debug: Checking file: ${filePath}`)
-      
-      if (!existsSync(filePath)) {
+
+      if (!fs.existsSync(filePath)) {
         console.log(`❌ Missing critical file: ${file} at ${filePath}`)
         return false
       }
       
       // Verify size for critical files
       if (CRITICAL_MODEL_CONFIG.modelSize[file]) {
-        const stats = await stat(filePath)
+        const stats = await fsPromises.stat(filePath)
         const expectedSize = CRITICAL_MODEL_CONFIG.modelSize[file]
         
         if (Math.abs(stats.size - expectedSize) > 1000) { // Allow 1KB variance
@@ -263,24 +271,37 @@ export class ModelGuardian {
    * Detect where models should be stored
    */
   private detectModelPath(): string {
-    const candidates = [
-      process.env.BRAINY_MODELS_PATH,
-      './models',
-      join(process.cwd(), 'models'),
-      join(process.env.HOME || '', '.brainy', 'models'),
-      '/opt/models', // Lambda/container path
-      env.cacheDir
-    ]
-    
-    for (const path of candidates) {
-      if (path && existsSync(path)) {
-        const modelPath = join(path, ...CRITICAL_MODEL_CONFIG.modelName.split('/'))
-        if (existsSync(join(modelPath, 'onnx', 'model.onnx'))) {
-          return path // Return the models directory, not its parent
+    // Browser always uses default path
+    if (typeof window !== 'undefined') {
+      return './models'
+    }
+
+    // Use require for synchronous access in Node.js
+    try {
+      const fs = require('node:fs')
+      const path = require('node:path')
+
+      const candidates = [
+        process.env.BRAINY_MODELS_PATH,
+        './models',
+        path.join(process.cwd(), 'models'),
+        path.join(process.env.HOME || '', '.brainy', 'models'),
+        '/opt/models', // Lambda/container path
+        env.cacheDir
+      ]
+
+      for (const candidatePath of candidates) {
+        if (candidatePath && fs.existsSync(candidatePath)) {
+          const modelPath = path.join(candidatePath, ...CRITICAL_MODEL_CONFIG.modelName.split('/'))
+          if (fs.existsSync(path.join(modelPath, 'onnx', 'model.onnx'))) {
+            return candidatePath // Return the models directory, not its parent
+          }
         }
       }
+    } catch (e) {
+      // If Node.js modules not available, return default
     }
-    
+
     // Default
     return './models'
   }