Polish the migration

src/lib/core/backup.ts

@@ -0,0 +1,83 @@
+import type {Settings} from '$lib/types/settings';
+
+export interface HistoryBackupPayload {
+    history: unknown[];
+    bookmarks: unknown[];
+    markers: unknown[];
+    readLog: unknown[];
+    readingStats: Record<string, unknown>;
+    dailyReadCounts: Record<string, number>;
+}
+
+export interface AppDataBackup {
+    version: 1;
+    exportedAt: string;
+    settings: Settings;
+    history: HistoryBackupPayload;
+}
+
+function isObject(value: unknown): value is Record<string, unknown> {
+    return typeof value === 'object' && value !== null;
+}
+
+export function buildAppDataBackup(settings: Settings, history: HistoryBackupPayload): AppDataBackup {
+    return {
+        version: 1,
+        exportedAt: new Date().toISOString(),
+        settings,
+        history,
+    };
+}
+
+export function parseAppDataBackup(raw: string): AppDataBackup {
+    const parsed = JSON.parse(raw) as unknown;
+    if (!isObject(parsed)) throw new Error('Backup file is not a valid object');
+    if (parsed.version !== 1) throw new Error('Unsupported backup format version');
+    if (!isObject(parsed.settings)) throw new Error('Backup is missing settings data');
+    if (!isObject(parsed.history)) throw new Error('Backup is missing history data');
+
+    const history = parsed.history;
+
+    return {
+        version: 1,
+        exportedAt: typeof parsed.exportedAt === 'string' ? parsed.exportedAt : new Date().toISOString(),
+        settings: parsed.settings as unknown as Settings,
+        history: {
+            history: Array.isArray(history.history) ? history.history : [],
+            bookmarks: Array.isArray(history.bookmarks) ? history.bookmarks : [],
+            markers: Array.isArray(history.markers) ? history.markers : [],
+            readLog: Array.isArray(history.readLog) ? history.readLog : [],
+            readingStats: isObject(history.readingStats) ? history.readingStats : {},
+            dailyReadCounts: isObject(history.dailyReadCounts) ? (history.dailyReadCounts as Record<string, number>) : {},
+        },
+    };
+}
+
+export function downloadAppDataBackup(backup: AppDataBackup, filename = 'moku-app-backup.json'): void {
+    const blob = new Blob([JSON.stringify(backup, null, 2)], {type: 'application/json'});
+    const url = URL.createObjectURL(blob);
+
+    const anchor = document.createElement('a');
+    anchor.href = url;
+    anchor.download = filename;
+    document.body.appendChild(anchor);
+    anchor.click();
+    document.body.removeChild(anchor);
+
+    setTimeout(() => URL.revokeObjectURL(url), 1000);
+}
+
+export function pickAppDataBackupFile(): Promise<File | null> {
+    return new Promise((resolve) => {
+        const input = document.createElement('input');
+        input.type = 'file';
+        input.accept = '.json,application/json';
+
+        input.addEventListener('change', () => {
+            const file = input.files?.[0] ?? null;
+            resolve(file);
+        }, {once: true});
+
+        input.click();
+    });
+}