feat: add admin panel, Replicate AI translation, and document translation

- Admin panel (/admin) with JWT auth: configure Replicate API token,
  JigsawStack API key, model version, enable/disable AI translation,
  change admin password. Settings persisted in data/settings.json.

- Replicate AI translation: POST /api/translate/replicate uses
  JigsawStack text-translate model via Replicate API. Main page
  switches to client-side AI translation when enabled.

- Document translation tab: supports PDF, DOCX, XLSX, XLS, CSV.
  Excel/Word formatting fully preserved (SheetJS + JSZip XML manipulation).
  PDF uses pdf-parse extraction + pdf-lib reconstruction.
  Column selector UI for tabular data (per-sheet, All/None toggles).

- Updated README with full implementation documentation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

utils/settings-store.ts

@@ -0,0 +1,48 @@
+import fs from "fs";
+import path from "path";
+
+export type Settings = {
+    replicateApiToken: string;
+    jigsawApiKey: string;
+    modelVersion: string;
+    replicateEnabled: boolean;
+    adminPasswordHash: string;
+};
+
+const DEFAULT_SETTINGS: Settings = {
+    replicateApiToken: process.env["REPLICATE_API_TOKEN"] ?? "",
+    jigsawApiKey: process.env["JIGSAWSTACK_API_KEY"] ?? "",
+    modelVersion: "jigsawstack/text-translate:454df4c49941c05dea05175bd37686d0872c73c1f9366d1c2505db32ade52a89",
+    replicateEnabled: false,
+    adminPasswordHash: process.env["ADMIN_PASSWORD"] ?? "admin"
+};
+
+const SETTINGS_PATH = path.join(process.cwd(), "data", "settings.json");
+
+function ensureDataDir() {
+    const dir = path.dirname(SETTINGS_PATH);
+    if (!fs.existsSync(dir)) {
+        fs.mkdirSync(dir, { recursive: true });
+    }
+}
+
+export function readSettings(): Settings {
+    try {
+        ensureDataDir();
+        if (!fs.existsSync(SETTINGS_PATH)) {
+            return { ...DEFAULT_SETTINGS };
+        }
+        const raw = fs.readFileSync(SETTINGS_PATH, "utf-8");
+        return { ...DEFAULT_SETTINGS, ...JSON.parse(raw) };
+    } catch {
+        return { ...DEFAULT_SETTINGS };
+    }
+}
+
+export function writeSettings(updates: Partial<Settings>): Settings {
+    ensureDataDir();
+    const current = readSettings();
+    const next = { ...current, ...updates };
+    fs.writeFileSync(SETTINGS_PATH, JSON.stringify(next, null, 2), "utf-8");
+    return next;
+}