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

config/markbase.toml

@@ -0,0 +1,41 @@
+[server]
+host = "127.0.0.1"
+port = 11438
+log_level = "info"
+auth_db_path = "data/auth.sqlite"
+users_db_dir = "data/users"
+
+[postgresql]
+host = "127.0.0.1"
+port = 5432
+user = "sftpgo"
+password = "sftpgo_pass_2026"
+database = "sftpgo"
+connection_pool_size = 5
+
+[authentication]
+bcrypt_cost = 10
+token_validity_hours = 24
+session_storage = "memory"
+max_sessions_per_user = 5
+default_user = "demo"
+default_password = "demo123"
+
+[test]
+users = [
+    "warren",
+    "momentry",
+    "demo",
+]
+password = "demo123"
+login_test_iterations = 10
+verify_test_iterations = 100
+api_test_iterations = 50
+performance_report = true
+output_format = "markdown"
+
+[logging]
+level = "info"
+file_path = "logs/markbase.log"
+console_output = true
+structured_logging = false