Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/services/processingCache.ts

@@ -0,0 +1,138 @@
+import { ProcessedFile, CacheConfig, CacheEntry, CacheStats } from '@app/types/processing';
+
+export class ProcessingCache {
+  private cache = new Map<string, CacheEntry>();
+  private totalSize = 0;
+  
+  constructor(private config: CacheConfig = {
+    maxFiles: 20,
+    maxSizeBytes: 2 * 1024 * 1024 * 1024, // 2GB
+    ttlMs: 30 * 60 * 1000 // 30 minutes
+  }) {}
+
+  set(key: string, data: ProcessedFile): void {
+    // Remove expired entries first
+    this.cleanup();
+    
+    // Calculate entry size (rough estimate)
+    const size = this.calculateSize(data);
+    
+    // Make room if needed
+    this.makeRoom(size);
+    
+    this.cache.set(key, {
+      data,
+      size,
+      lastAccessed: Date.now(),
+      createdAt: Date.now()
+    });
+    
+    this.totalSize += size;
+  }
+
+  get(key: string): ProcessedFile | null {
+    const entry = this.cache.get(key);
+    if (!entry) return null;
+    
+    // Check TTL
+    if (Date.now() - entry.createdAt > this.config.ttlMs) {
+      this.delete(key);
+      return null;
+    }
+    
+    // Update last accessed
+    entry.lastAccessed = Date.now();
+    return entry.data;
+  }
+
+  has(key: string): boolean {
+    const entry = this.cache.get(key);
+    if (!entry) return false;
+    
+    // Check TTL
+    if (Date.now() - entry.createdAt > this.config.ttlMs) {
+      this.delete(key);
+      return false;
+    }
+    
+    return true;
+  }
+
+  private makeRoom(neededSize: number): void {
+    // Remove oldest entries until we have space
+    while (
+      this.cache.size >= this.config.maxFiles ||
+      this.totalSize + neededSize > this.config.maxSizeBytes
+    ) {
+      const oldestKey = this.findOldestEntry();
+      if (oldestKey) {
+        this.delete(oldestKey);
+      } else break;
+    }
+  }
+
+  private findOldestEntry(): string | null {
+    let oldest: { key: string; lastAccessed: number } | null = null;
+    
+    for (const [key, entry] of this.cache) {
+      if (!oldest || entry.lastAccessed < oldest.lastAccessed) {
+        oldest = { key, lastAccessed: entry.lastAccessed };
+      }
+    }
+    
+    return oldest?.key || null;
+  }
+
+  private cleanup(): void {
+    const now = Date.now();
+    for (const [key, entry] of this.cache) {
+      if (now - entry.createdAt > this.config.ttlMs) {
+        this.delete(key);
+      }
+    }
+  }
+
+  private calculateSize(data: ProcessedFile): number {
+    // Rough size estimation
+    let size = 0;
+    
+    // Estimate size of thumbnails (main memory consumer)
+    data.pages.forEach(page => {
+      if (page.thumbnail) {
+        // Base64 thumbnail is roughly 50KB each
+        size += 50 * 1024;
+      }
+    });
+    
+    // Add some overhead for other data
+    size += 10 * 1024; // 10KB overhead
+    
+    return size;
+  }
+
+  delete(key: string): void {
+    const entry = this.cache.get(key);
+    if (entry) {
+      this.totalSize -= entry.size;
+      this.cache.delete(key);
+    }
+  }
+
+  clear(): void {
+    this.cache.clear();
+    this.totalSize = 0;
+  }
+
+  getStats(): CacheStats {
+    return {
+      entries: this.cache.size,
+      totalSizeBytes: this.totalSize,
+      maxSizeBytes: this.config.maxSizeBytes
+    };
+  }
+
+  // Get all cached keys (for debugging and cleanup)
+  getKeys(): string[] {
+    return Array.from(this.cache.keys());
+  }
+}