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/preferencesService.ts

@@ -0,0 +1,86 @@
+import { type ToolPanelMode, DEFAULT_TOOL_PANEL_MODE } from '@app/constants/toolPanel';
+import { type ThemeMode, getSystemTheme } from '@app/constants/theme';
+
+export interface UserPreferences {
+  autoUnzip: boolean;
+  autoUnzipFileLimit: number;
+  defaultToolPanelMode: ToolPanelMode;
+  theme: ThemeMode;
+  toolPanelModePromptSeen: boolean;
+  showLegacyToolDescriptions: boolean;
+  hasCompletedOnboarding: boolean;
+}
+
+export const DEFAULT_PREFERENCES: UserPreferences = {
+  autoUnzip: true,
+  autoUnzipFileLimit: 4,
+  defaultToolPanelMode: DEFAULT_TOOL_PANEL_MODE,
+  theme: getSystemTheme(),
+  toolPanelModePromptSeen: false,
+  showLegacyToolDescriptions: false,
+  hasCompletedOnboarding: false,
+};
+
+const STORAGE_KEY = 'stirlingpdf_preferences';
+
+class PreferencesService {
+  getPreference<K extends keyof UserPreferences>(
+    key: K
+  ): UserPreferences[K] {
+    // Explicitly re-read every time in case preferences have changed in another tab etc.
+    try {
+      const stored = localStorage.getItem(STORAGE_KEY);
+      if (stored) {
+        const preferences = JSON.parse(stored) as Partial<UserPreferences>;
+        if (key in preferences && preferences[key] !== undefined) {
+          return preferences[key]!;
+        }
+      }
+    } catch (error) {
+      console.error('Error reading preference:', key, error);
+    }
+    return DEFAULT_PREFERENCES[key];
+  }
+
+  setPreference<K extends keyof UserPreferences>(
+    key: K,
+    value: UserPreferences[K]
+  ): void {
+    try {
+      const stored = localStorage.getItem(STORAGE_KEY);
+      const preferences = stored ? JSON.parse(stored) : {};
+      preferences[key] = value;
+      localStorage.setItem(STORAGE_KEY, JSON.stringify(preferences));
+    } catch (error) {
+      console.error('Error writing preference:', key, error);
+    }
+  }
+
+  getAllPreferences(): UserPreferences {
+    try {
+      const stored = localStorage.getItem(STORAGE_KEY);
+      if (stored) {
+        const preferences = JSON.parse(stored) as Partial<UserPreferences>;
+        // Merge with defaults to ensure all preferences exist
+        return {
+          ...DEFAULT_PREFERENCES,
+          ...preferences,
+        };
+      }
+    } catch (error) {
+      console.error('Error reading preferences', error);
+    }
+    return { ...DEFAULT_PREFERENCES };
+  }
+
+  clearAllPreferences(): void {
+    try {
+      localStorage.removeItem(STORAGE_KEY);
+    } catch (error) {
+      console.error('Error clearing preferences:', error);
+      throw error;
+    }
+  }
+}
+
+export const preferencesService = new PreferencesService();