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

src/main.rs

@@ -1,4 +1,5 @@
 use clap::{Parser, Subcommand};
+use std::path::Path;
 
 #[derive(Parser)]
 #[command(name = "markbase", about = "Momentry Display Engine")]
@@ -22,6 +23,34 @@ enum Commands {
         #[arg(short, long)]
         output: Option<String>,
     },
+    /// Configuration management
+    Config {
+        #[command(subcommand)]
+        action: ConfigCommands,
+    },
+}
+
+#[derive(Subcommand)]
+enum ConfigCommands {
+    /// Initialize default configuration file
+    Init {
+        #[arg(short, long)]
+        force: bool,
+    },
+    /// Show current configuration
+    Show {
+        #[arg(short, long)]
+        section: Option<String>,
+    },
+    /// Edit configuration
+    Edit {
+        #[arg(short, long)]
+        key: String,
+        #[arg(short, long)]
+        value: String,
+    },
+    /// Validate configuration
+    Validate,
 }
 
 #[tokio::main]
@@ -41,6 +70,103 @@ async fn main() -> anyhow::Result<()> {
                 println!("{html}");
             }
         }
+        Commands::Config { action } => {
+            handle_config_command(action)?;
+        }
     }
     Ok(())
 }
+
+fn handle_config_command(action: ConfigCommands) -> anyhow::Result<()> {
+    match action {
+        ConfigCommands::Init { force } => {
+            let config_path = Path::new("config/markbase.toml");
+            
+            if config_path.exists() && !force {
+                println!("Configuration file already exists at config/markbase.toml");
+                println!("Use --force to overwrite");
+                return Ok(());
+            }
+            
+            let config = markbase::config::MarkBaseConfig::default_config();
+            config.save(config_path)?;
+            
+            println!("✓ Configuration file created: config/markbase.toml");
+            println!("Default values:");
+            println!("  Server port: {}", config.server.port);
+            println!("  PostgreSQL host: {}", config.postgresql.host);
+            println!("  Test users: {}", config.test.users.join(", "));
+        }
+        ConfigCommands::Show { section } => {
+            let config_path = Path::new("config/markbase.toml");
+            
+            if !config_path.exists() {
+                println!("Configuration file not found. Run 'markbase config init' first.");
+                return Ok(());
+            }
+            
+            let config = markbase::config::MarkBaseConfig::load(config_path)?;
+            
+            if let Some(s) = section {
+                show_section(&config, &s);
+            } else {
+                println!("{}", toml::to_string_pretty(&config)?);
+            }
+        }
+        ConfigCommands::Edit { key, value } => {
+            let config_path = Path::new("config/markbase.toml");
+            
+            if !config_path.exists() {
+                println!("Configuration file not found. Run 'markbase config init' first.");
+                return Ok(());
+            }
+            
+            let mut config = markbase::config::MarkBaseConfig::load(config_path)?;
+            
+            match config.get(&key) {
+                Some(old_value) => {
+                    config.set(&key, &value)?;
+                    config.validate()?;
+                    config.save(config_path)?;
+                    println!("✓ Updated {}: {} → {}", key, old_value, value);
+                }
+                None => {
+                    println!("Invalid config key: {}", key);
+                    println!("Valid keys: server.*, postgresql.*, authentication.*, test.*, logging.*");
+                }
+            }
+        }
+        ConfigCommands::Validate => {
+            let config_path = Path::new("config/markbase.toml");
+            
+            if !config_path.exists() {
+                println!("Configuration file not found. Run 'markbase config init' first.");
+                return Ok(());
+            }
+            
+            let config = markbase::config::MarkBaseConfig::load(config_path)?;
+            
+            match config.validate() {
+                Ok(_) => {
+                    println!("✓ Configuration is valid");
+                }
+                Err(e) => {
+                    println!("✗ Configuration validation failed: {}", e);
+                }
+            }
+        }
+    }
+    
+    Ok(())
+}
+
+fn show_section(config: &markbase::config::MarkBaseConfig, section: &str) {
+    match section {
+        "server" => println!("{}", toml::to_string_pretty(&config.server).unwrap()),
+        "postgresql" => println!("{}", toml::to_string_pretty(&config.postgresql).unwrap()),
+        "authentication" => println!("{}", toml::to_string_pretty(&config.authentication).unwrap()),
+        "test" => println!("{}", toml::to_string_pretty(&config.test).unwrap()),
+        "logging" => println!("{}", toml::to_string_pretty(&config.logging).unwrap()),
+        _ => println!("Invalid section: {}. Valid sections: server, postgresql, authentication, test, logging", section),
+    }
+}