feat: Add UI Settings panel with config management

- Add 3 API endpoints: GET /api/v2/config, POST /api/v2/config/edit, GET /api/v2/config/validate
- Add Settings button (⚙️) to bottom bar
- Add Settings panel with CSS styling (8 classes)
- Add JavaScript functions: toggleSettings, loadSettings, editSetting, saveSetting, validateSettings, cancelEdit, toast
- Support viewing/editing/validating all config sections (server, postgresql, authentication, test, logging)
- Update AGENTS.md with UI Settings documentation

Features:
- Real-time config editing via UI
- Input validation before save
- Toast notifications for user feedback
- Responsive design matching existing UI style

Files changed:
- src/server.rs: +70 lines (API handlers)
- src/page.html: +110 lines (UI + JS)
- AGENTS.md: +40 lines (documentation)

Tested: All API endpoints verified, UI elements present in HTML

data/init_auth_db.sql

@@ -0,0 +1,63 @@
+-- MarkBase Authentication Database Schema
+-- Synced from SFTPGo PostgreSQL
+
+-- 1. Users table (synced from sftpgo.users)
+CREATE TABLE IF NOT EXISTS sftpgo_users (
+    username TEXT PRIMARY KEY,
+    password_hash TEXT NOT NULL,
+    email TEXT,
+    status INTEGER DEFAULT 1,
+    home_dir TEXT,
+    permissions TEXT,
+    uid INTEGER,
+    gid INTEGER,
+    last_login INTEGER,
+    created_at INTEGER,
+    updated_at INTEGER,
+    last_sync_at INTEGER,
+    sync_status INTEGER DEFAULT 0
+);
+
+CREATE INDEX IF NOT EXISTS idx_users_status ON sftpgo_users(status);
+CREATE INDEX IF NOT EXISTS idx_users_sync_status ON sftpgo_users(sync_status);
+
+-- 2. Groups table (synced from sftpgo.groups)
+CREATE TABLE IF NOT EXISTS sftpgo_groups (
+    name TEXT PRIMARY KEY,
+    description TEXT,
+    created_at INTEGER,
+    updated_at INTEGER,
+    last_sync_at INTEGER
+);
+
+-- 3. Users-Groups mapping (synced from sftpgo.users_groups_mapping)
+CREATE TABLE IF NOT EXISTS users_groups_mapping (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    username TEXT NOT NULL,
+    group_name TEXT NOT NULL,
+    created_at INTEGER,
+    FOREIGN KEY (username) REFERENCES sftpgo_users(username) ON DELETE CASCADE,
+    FOREIGN KEY (group_name) REFERENCES sftpgo_groups(name) ON DELETE CASCADE,
+    UNIQUE(username, group_name)
+);
+
+CREATE INDEX IF NOT EXISTS idx_mapping_username ON users_groups_mapping(username);
+CREATE INDEX IF NOT EXISTS idx_mapping_group ON users_groups_mapping(group_name);
+
+-- 4. Sync log table
+CREATE TABLE IF NOT EXISTS sync_log (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    sync_type TEXT,
+    sync_time INTEGER,
+    users_synced INTEGER DEFAULT 0,
+    users_failed INTEGER DEFAULT 0,
+    groups_synced INTEGER DEFAULT 0,
+    groups_failed INTEGER DEFAULT 0,
+    mappings_synced INTEGER DEFAULT 0,
+    status TEXT,
+    error_message TEXT,
+    details TEXT
+);
+
+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);