feat: Add admin authentication for Settings panel

- Add sftpgo_admins table to auth.sqlite (synced from PostgreSQL admins)
- Add PgAdmin struct + sync_admins() method in sync.rs
- Add fetch_admins() method in pg_client.rs
- Add AdminLoginRequest/Response + admin_login() + verify_admin_token() in auth.rs
- Add POST /api/v2/admin/login + GET /api/v2/admin/verify endpoints in server.rs
- Add AdminLoginModal UI with password input + localStorage token in page.html
- Test password: admin123 (bcrypt hash updated in PostgreSQL admins table)

Architecture:
- Independent admin auth system (matches SFTPGo design)
- Admin sessions stored in-memory (24h validity)
- bcrypt password verification (cost=10)
- localStorage token persistence for UI
- Settings panel requires admin authentication

Files changed:
- data/init_auth_db.sql: +20 lines
- src/sync.rs: +100 lines
- src/pg_client.rs: +50 lines
- src/auth.rs: +60 lines
- src/server.rs: +50 lines
- src/page.html: +70 lines
Total: ~290 lines added

Tested: Admin sync, login, verify, UI modal all working

data/init_auth_db.sql

@@ -61,3 +61,21 @@ CREATE TABLE IF NOT EXISTS sync_log (
 
 CREATE INDEX IF NOT EXISTS idx_sync_time ON sync_log(sync_time);
 CREATE INDEX IF NOT EXISTS idx_sync_status ON sync_log(status);
+
+-- 5. Admins table (synced from sftpgo.admins)
+CREATE TABLE IF NOT EXISTS sftpgo_admins (
+    username TEXT PRIMARY KEY,
+    password_hash TEXT NOT NULL,
+    email TEXT,
+    description TEXT,
+    status INTEGER DEFAULT 1,
+    permissions TEXT NOT NULL,
+    filters TEXT,
+    role_id INTEGER,
+    last_login INTEGER DEFAULT 0,
+    created_at INTEGER NOT NULL,
+    updated_at INTEGER NOT NULL,
+    last_sync_at INTEGER
+);
+
+CREATE INDEX IF NOT EXISTS idx_admins_status ON sftpgo_admins(status);