feat(ThemeManager): create ThemeManager that uses persistent storage to store preferred user theme

src/features/ChangeAppTheme/model/store/ThemeManager/ThemeManager.svelte.ts

@@ -0,0 +1,99 @@
+import { createPersistentStore } from '$shared/lib';
+
+type Theme = 'light' | 'dark';
+type ThemeSource = 'system' | 'user';
+
+class ThemeManager {
+    // Private reactive state
+    #theme = $state<Theme>('light');
+    #source = $state<ThemeSource>('system');
+    #mediaQuery: MediaQueryList | null = null;
+    #store = createPersistentStore<Theme | null>('glyphdiff:theme', null);
+    #systemChangeHandler = this.#onSystemChange.bind(this);
+
+    constructor() {
+        // Derive initial values from stored preference or OS
+        const stored = this.#store.value;
+        if (stored === 'dark' || stored === 'light') {
+            this.#theme = stored;
+            this.#source = 'user';
+        } else {
+            this.#theme = this.#getSystemTheme();
+            this.#source = 'system';
+        }
+    }
+
+    get value(): Theme {
+        return this.#theme;
+    }
+
+    get source(): ThemeSource {
+        return this.#source;
+    }
+
+    get isDark(): boolean {
+        return this.#theme === 'dark';
+    }
+
+    get isUserControlled(): boolean {
+        return this.#source === 'user';
+    }
+
+    /** Call once in root onMount */
+    init(): void {
+        this.#applyToDom(this.#theme);
+        this.#mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
+        this.#mediaQuery.addEventListener('change', this.#systemChangeHandler);
+    }
+
+    /** Call in root onDestroy */
+    destroy(): void {
+        this.#mediaQuery?.removeEventListener('change', this.#systemChangeHandler);
+        this.#mediaQuery = null;
+    }
+
+    setTheme(theme: Theme): void {
+        this.#source = 'user';
+        this.#theme = theme;
+        this.#store.value = theme;
+        this.#applyToDom(theme);
+    }
+
+    toggle(): void {
+        this.setTheme(this.value === 'dark' ? 'light' : 'dark');
+    }
+
+    /** Hand control back to OS */
+    resetToSystem(): void {
+        this.#store.clear();
+        this.#theme = this.#getSystemTheme();
+        this.#source = 'system';
+        this.#applyToDom(this.#theme);
+    }
+
+    // -------------------------
+    // Private helpers
+    // -------------------------
+
+    #getSystemTheme(): Theme {
+        if (typeof window === 'undefined') {
+            return 'light';
+        }
+
+        return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+    }
+
+    #applyToDom(theme: Theme): void {
+        document.documentElement.classList.toggle('dark', theme === 'dark');
+    }
+
+    #onSystemChange(e: MediaQueryListEvent): void {
+        if (this.#source === 'system') {
+            this.#theme = e.matches ? 'dark' : 'light';
+            this.#applyToDom(this.#theme);
+        }
+    }
+}
+
+// Export a singleton — one instance for the whole app
+export const themeManager = new ThemeManager();