feat(backup): autosave backend, continuous snapshots, and Settings modal

Background autosave toggle plus whole-book snapshots (default <book>/Folio-Backups, configurable location, retention, interval) with a one-minute change check in the main process. New Settings modal (sidebar, Ctrl+, , File menu) exposes autosave, snapshots, folder picker, and Snapshot now.

tests/run-snapshots-test.mjs

@@ -0,0 +1,99 @@
+// Standalone test for continuous snapshot logic (src/main/backup.ts).
+// Run with: node tests/run-snapshots-test.mjs
+import fs from "fs";
+import os from "os";
+import path from "path";
+import { bundleForTest } from "./helpers/bundle.mjs";
+
+const { mod: project, cleanupBundle: cleanupProj } = await bundleForTest({
+  name: "folio-snap-project",
+  entryPoints: ["src/main/project.ts"],
+});
+const { mod: chapters, cleanupBundle: cleanupChap } = await bundleForTest({
+  name: "folio-snap-chapters",
+  entryPoints: ["src/main/chapters.ts"],
+});
+const { mod: backup, cleanupBundle: cleanupBackup } = await bundleForTest({
+  name: "folio-snap-backup",
+  entryPoints: ["src/main/backup.ts"],
+});
+
+const { createBook } = project;
+const { createChapter, setChapterContent } = chapters;
+const {
+  bindBackupSettings,
+  getSnapshotRoot,
+  listSnapshots,
+  createSnapshot,
+  pruneSnapshots,
+  bookChangedSince,
+  SNAPSHOT_DIR_NAME,
+} = backup;
+
+let failures = 0;
+function ok(cond, msg) {
+  if (cond) console.log("  ok -", msg);
+  else { failures++; console.error("FAIL -", msg); }
+}
+
+// In-memory settings store (backup.ts stays Electron-free).
+const settings = new Map();
+bindBackupSettings({
+  get: (k, fb) => (settings.has(k) ? settings.get(k) : fb),
+  set: (k, v) => { settings.set(k, v); },
+});
+
+const dir = fs.mkdtempSync(path.join(os.tmpdir(), "folio-snap-"));
+createBook(dir, "Snap Book");
+const c = createChapter(dir, "One");
+setChapterContent(dir, c.id, "hello");
+
+// default root = <book>/Folio-Backups
+ok(getSnapshotRoot(dir) === path.join(dir, SNAPSHOT_DIR_NAME), "default snapshot root is per-book folder");
+
+// snapshot creates a timestamped copy with content
+const s1 = createSnapshot(dir, 20);
+ok(typeof s1 === "string" && fs.existsSync(s1), "createSnapshot returns existing dir: " + s1);
+ok(fs.existsSync(path.join(s1, "folio.json")), "snapshot contains folio.json");
+ok(listSnapshots(dir).length === 1, "one snapshot listed");
+
+// snapshots never nest: second snapshot must not contain the backup dir
+await new Promise((r) => setTimeout(r, 5));
+const s2 = createSnapshot(dir, 20);
+ok(listSnapshots(dir).length === 2, "two snapshots listed");
+ok(!fs.existsSync(path.join(s2, SNAPSHOT_DIR_NAME)), "snapshot does not contain nested backup dir");
+
+// bookChangedSince: quiet -> false, touched -> true
+ok(bookChangedSince(dir, Date.now()) === false, "no change since now");
+setChapterContent(dir, c.id, "hello edited");
+ok(bookChangedSince(dir, Date.now() - 1000) === true, "detects content change");
+
+// pruning keeps newest N
+await new Promise((r) => setTimeout(r, 5));
+createSnapshot(dir, 20);
+ok(listSnapshots(dir).length === 3, "three snapshots before prune");
+pruneSnapshots(dir, 2);
+const kept = listSnapshots(dir);
+ok(kept.length === 2, "prune keeps newest 2");
+ok(!fs.existsSync(s1), "oldest snapshot pruned");
+
+// custom root honored
+const custom = fs.mkdtempSync(path.join(os.tmpdir(), "folio-snaproot-"));
+settings.set("snapshotLocation", custom);
+ok(getSnapshotRoot(dir) === custom, "custom snapshot root honored");
+const s3 = createSnapshot(dir, 20);
+ok(s3.startsWith(custom), "snapshot lands in custom root");
+settings.delete("snapshotLocation");
+
+// missing dir -> null, no throw
+ok(createSnapshot(path.join(dir, "nope"), 5) === null, "missing book returns null");
+
+// cleanup
+fs.rmSync(dir, { recursive: true, force: true });
+fs.rmSync(custom, { recursive: true, force: true });
+cleanupProj();
+cleanupChap();
+cleanupBackup();
+
+if (failures) { console.error(`${failures} snapshot check(s) FAILED`); process.exit(1); }
+console.log("All snapshot checks passed.");