✨ feat(config): implement configuration file management

- add config.rs to handle program configuration
- create default configuration with retention rules
- implement save and load functions for persistent storage
- add credential file setting

src/config.rs

@@ -0,0 +1,100 @@
+use std::{
+    env,
+    fs::{self, read_to_string},
+    path::PathBuf,
+};
+
+use serde::{Deserialize, Serialize};
+
+mod eol_rule;
+
+use eol_rule::EolRule;
+
+use crate::{Error, MessageAge, Retention};
+
+/// Configuration file for the program
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Config {
+    credentials: Option<String>,
+    rules: Vec<EolRule>,
+}
+
+impl Default for Config {
+    fn default() -> Self {
+        let rules = Vec::new();
+
+        let mut cfg = Self {
+            credentials: Some("credential.json".to_string()),
+            rules,
+        };
+
+        cfg.add_rule(Retention::new(MessageAge::Years(1), true))
+            .add_rule(Retention::new(MessageAge::Weeks(1), true))
+            .add_rule(Retention::new(MessageAge::Months(1), true))
+            .add_rule(Retention::new(MessageAge::Years(5), true));
+
+        cfg
+    }
+}
+
+impl Config {
+    /// Create a new configuration file
+    pub fn new() -> Self {
+        Config::default()
+    }
+
+    /// Set a name for the credentials file
+    pub fn set_credentials(&mut self, file_name: &str) -> &mut Self {
+        self.credentials = Some(file_name.to_string());
+        self
+    }
+
+    /// Add a new rule to the rule set by setting the retention age
+    pub fn add_rule(&mut self, retention: Retention) -> &mut Self {
+        let id = if let Some(max) = self.rules.iter().max_by_key(|r| r.id()) {
+            max.id() + 1
+        } else {
+            1
+        };
+
+        let mut rule = EolRule::new(id);
+        rule.set_retention(retention);
+        self.rules.push(rule);
+        self
+    }
+
+    /// Save the current configuration to the file
+    pub fn save(&self) -> Result<(), Error> {
+        let home_dir = env::home_dir().unwrap();
+        let path = PathBuf::new()
+            .join(home_dir)
+            .join(".cull-gmail/cull-gmail.toml");
+
+        if let Ok(output) = toml::to_string(self) {
+            fs::write(path, output)?;
+        }
+
+        Ok(())
+    }
+
+    /// Load the current configuration
+    pub fn load() -> Result<Config, Error> {
+        let home_dir = env::home_dir().unwrap();
+        let path = PathBuf::new()
+            .join(home_dir)
+            .join(".cull-gmail/cull-gmail.toml");
+
+        let input = read_to_string(path)?;
+        let config = toml::from_str::<Config>(&input)?;
+        Ok(config)
+    }
+
+    /// Return the credential file name
+    pub fn credential_file(&self) -> &str {
+        if let Some(file) = &self.credentials {
+            file
+        } else {
+            ""
+        }
+    }
+}