feat: Add configurable CEF command line flags (#16)

## Summary
- Add CommandLineConfig struct for CEF command line switches
- Use direct struct initialization with optional helper methods
- Change default to secure: only use-mock-keychain enabled on macOS debug builds
- Add comprehensive documentation with usage examples

## Usage
```rust
use bevy_cef::prelude::*;

// Default (secure, includes use-mock-keychain on macOS debug)
app.add_plugins((DefaultPlugins, CefPlugin::default()));

// Add switches while preserving defaults (recommended)
app.add_plugins((
    DefaultPlugins,
    CefPlugin {
        command_line_config: CommandLineConfig::default()
            .with_switch("disable-gpu")
            .with_switch_value("remote-debugging-port", "9222"),
    },
));

// Full customization with direct initialization
app.add_plugins((
    DefaultPlugins,
    CefPlugin {
        command_line_config: CommandLineConfig {
            switches: vec!["disable-gpu"],
            switch_values: vec![("remote-debugging-port", "9222")],
        },
    },
));
```

crates/bevy_cef_core/src/browser_process.rs

@@ -2,6 +2,7 @@ mod app;
 mod browser_process_handler;
 mod browsers;
 mod client_handler;
+mod command_line_config;
 mod context_menu_handler;
 mod display_handler;
 mod localhost;
@@ -13,6 +14,7 @@ pub use app::*;
 pub use browser_process_handler::*;
 pub use browsers::*;
 pub use client_handler::*;
+pub use command_line_config::*;
 pub use context_menu_handler::*;
 pub use localhost::*;
 pub use message_pump::*;

crates/bevy_cef_core/src/browser_process/app.rs

@@ -1,3 +1,4 @@
+use crate::browser_process::CommandLineConfig;
 use crate::browser_process::MessageLoopTimer;
 use crate::browser_process::browser_process_handler::BrowserProcessHandlerBuilder;
 use crate::util::{SCHEME_CEF, cef_scheme_flags};
@@ -15,13 +16,18 @@ use std::sync::mpsc::Sender;
 pub struct BrowserProcessAppBuilder {
     object: *mut RcImpl<_cef_app_t, Self>,
     message_loop_working_requester: Sender<MessageLoopTimer>,
+    config: CommandLineConfig,
 }
 
 impl BrowserProcessAppBuilder {
-    pub fn build(message_loop_working_requester: Sender<MessageLoopTimer>) -> cef::App {
+    pub fn build(
+        message_loop_working_requester: Sender<MessageLoopTimer>,
+        config: CommandLineConfig,
+    ) -> cef::App {
         cef::App::new(Self {
             object: core::ptr::null_mut(),
             message_loop_working_requester,
+            config,
         })
     }
 }
@@ -36,6 +42,7 @@ impl Clone for BrowserProcessAppBuilder {
         Self {
             object,
             message_loop_working_requester: self.message_loop_working_requester.clone(),
+            config: self.config.clone(),
         }
     }
 }
@@ -58,10 +65,14 @@ impl ImplApp for BrowserProcessAppBuilder {
         let Some(command_line) = command_line else {
             return;
         };
-        command_line.append_switch(Some(&"use-mock-keychain".into()));
-        // command_line.append_switch(Some(&"disable-gpu".into()));
-        // command_line.append_switch(Some(&"disable-gpu-compositing".into()));
-        // command_line.append_switch(Some(&" disable-gpu-shader-disk-cache".into()));
+
+        for switch in &self.config.switches {
+            command_line.append_switch(Some(&(*switch).into()));
+        }
+
+        for (name, value) in &self.config.switch_values {
+            command_line.append_switch_with_value(Some(&(*name).into()), Some(&(*value).into()));
+        }
     }
 
     fn on_register_custom_schemes(&self, registrar: Option<&mut SchemeRegistrar>) {

crates/bevy_cef_core/src/browser_process/command_line_config.rs

@@ -0,0 +1,56 @@
+/// Configuration for CEF command line switches.
+///
+/// Used to customize CEF behavior at startup.
+///
+/// # Default Switches
+///
+/// On macOS debug builds, the following switches are enabled by default:
+/// - `use-mock-keychain`: Uses a mock keychain for testing
+///
+/// # Example
+///
+/// ```no_run
+/// use bevy_cef::prelude::*;
+///
+/// // Add switches while preserving defaults (recommended)
+/// let config = CommandLineConfig::default()
+///     .with_switch("disable-gpu")
+///     .with_switch_value("remote-debugging-port", "9222");
+///
+/// // Or use direct initialization (replaces defaults)
+/// let config = CommandLineConfig {
+///     switches: vec!["disable-gpu"],
+///     switch_values: vec![("remote-debugging-port", "9222")],
+/// };
+/// ```
+#[derive(Clone, Debug)]
+pub struct CommandLineConfig {
+    pub switches: Vec<&'static str>,
+    pub switch_values: Vec<(&'static str, &'static str)>,
+}
+
+impl Default for CommandLineConfig {
+    fn default() -> Self {
+        Self {
+            switches: vec![
+                #[cfg(all(target_os = "macos", debug_assertions))]
+                "use-mock-keychain",
+            ],
+            switch_values: Vec::new(),
+        }
+    }
+}
+
+impl CommandLineConfig {
+    /// Add a command line switch (e.g., "disable-gpu", "disable-web-security").
+    pub fn with_switch(mut self, name: &'static str) -> Self {
+        self.switches.push(name);
+        self
+    }
+
+    /// Add a command line switch with a value (e.g., "remote-debugging-port", "9222").
+    pub fn with_switch_value(mut self, name: &'static str, value: &'static str) -> Self {
+        self.switch_values.push((name, value));
+        self
+    }
+}