preferences dialog with persistent settings

Add a preferences dialog (gear icon / Ctrl+,) with live-apply for:
- Font selection (via FontDialogButton)
- Color scheme (sourceview5 schemes dropdown)
- Line numbers, current line highlight, word wrap, tab width
- User-editable directory filter list (in a framed group)

Settings persist to ~/.config/meld-rs/settings.toml via serde/toml.
Uses a shared thread-local CssProvider for font CSS so updates
replace rather than stack. Fixes gutter arrow positioning with
word wrap by using per-line line_yrange instead of arithmetic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: joske

Cargo.lock

@@ -12,3 +12,5 @@ chrono = "0.4"
 notify = "8"
 clap = { version = "4", features = ["derive"] }
 sourceview5 = { version = "0.11.0", features = ["gtk_v4_14"] }
+serde = { version = "1", features = ["derive"] }
+toml = "0.8"

Cargo.toml

@@ -5,6 +5,7 @@ use gio::prelude::ApplicationExtManual;
 use gtk4::{Application, glib};
 
 mod myers;
+mod settings;
 mod ui;
 
 #[derive(Parser)]

src/main.rs

@@ -0,0 +1,79 @@
+use serde::{Deserialize, Serialize};
+use std::path::PathBuf;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(default)]
+pub struct Settings {
+    pub font: String,
+    pub style_scheme: String,
+    pub show_line_numbers: bool,
+    pub wrap_mode: String,
+    pub tab_width: u32,
+    pub highlight_current_line: bool,
+    pub dir_filters: Vec<String>,
+}
+
+impl Default for Settings {
+    fn default() -> Self {
+        Self {
+            font: "Monospace 11".to_string(),
+            style_scheme: "Adwaita".to_string(),
+            show_line_numbers: true,
+            wrap_mode: "none".to_string(),
+            tab_width: 4,
+            highlight_current_line: true,
+            dir_filters: vec![
+                ".git".into(),
+                ".svn".into(),
+                ".hg".into(),
+                ".bzr".into(),
+                "_darcs".into(),
+                ".CVS".into(),
+                "__pycache__".into(),
+                "node_modules".into(),
+                ".DS_Store".into(),
+            ],
+        }
+    }
+}
+
+impl Settings {
+    pub fn config_path() -> PathBuf {
+        let mut p = if let Some(config) = std::env::var_os("XDG_CONFIG_HOME") {
+            PathBuf::from(config)
+        } else if let Some(home) = std::env::var_os("HOME") {
+            PathBuf::from(home).join(".config")
+        } else {
+            PathBuf::from(".")
+        };
+        p.push("meld-rs");
+        p.push("settings.toml");
+        p
+    }
+
+    pub fn load() -> Self {
+        let path = Self::config_path();
+        match std::fs::read_to_string(&path) {
+            Ok(contents) => toml::from_str(&contents).unwrap_or_default(),
+            Err(_) => Self::default(),
+        }
+    }
+
+    pub fn save(&self) {
+        let path = Self::config_path();
+        if let Some(parent) = path.parent() {
+            let _ = std::fs::create_dir_all(parent);
+        }
+        if let Ok(contents) = toml::to_string_pretty(self) {
+            let _ = std::fs::write(&path, contents);
+        }
+    }
+
+    pub fn wrap_mode_gtk(&self) -> gtk4::WrapMode {
+        match self.wrap_mode.as_str() {
+            "word" => gtk4::WrapMode::Word,
+            "char" => gtk4::WrapMode::Char,
+            _ => gtk4::WrapMode::None,
+        }
+    }
+}