Add resolved appearance engine and paired theme settings

Resolve light, dark and automatic appearance centrally and publish live theme snapshots to native surfaces and the appearance portal. Add paired preset families, custom theme imports, accessibility transforms and contrast-safe transitions.

Refine the Settings gallery and Auto controls, move overrides under advanced customization, and share palette-aware switches and circular slider thumbs. Document configuration and include theme licenses and integration coverage.

shell/ferese-settings/src/store.rs

@@ -78,6 +78,7 @@ impl Snapshot {
 
     pub fn edit(&mut self, edit: &Edit) -> Result<(), String> {
         match edit {
+            Edit::Unset(path) => self.doc.unset(path),
             Edit::Set(path, value) => self.doc.set(path, value.clone()),
             Edit::Add(path, fields) => self.doc.add(path, fields.clone()),
             Edit::Remove(path, index) => self.doc.remove(path, *index),
@@ -90,6 +91,7 @@ impl Snapshot {
 
 #[derive(Clone, Debug)]
 pub enum Edit {
+    Unset(String),
     Set(String, Value),
     Add(String, Vec<(String, Value)>),
     Remove(String, usize),
@@ -108,6 +110,35 @@ pub fn config_path() -> PathBuf {
     path.canonicalize().unwrap_or(path)
 }
 
+pub fn pick_file(title: &str, filter: &str) -> Result<Option<String>, String> {
+    let mut debug = std::env::var("GDK_DEBUG").unwrap_or_default();
+    if !debug.is_empty() {
+        debug.push(',');
+    }
+    debug.push_str("no-portals");
+    let output = Command::new("zenity")
+        .env("GDK_DEBUG", debug)
+        .env("GTK_USE_PORTAL", "0")
+        .arg("--file-selection")
+        .arg(format!("--title={title}"))
+        .arg(format!("--file-filter={filter}"))
+        .arg("--file-filter=All files | *")
+        .output()
+        .map_err(|e| format!("Could not open the file chooser: {e}. Install zenity to choose files."))?;
+    if output.status.code() == Some(1) {
+        return Ok(None);
+    }
+    if !output.status.success() {
+        return Err(format!(
+            "File chooser failed: {}",
+            String::from_utf8_lossy(&output.stderr).trim()
+        ));
+    }
+    let path = String::from_utf8(output.stdout).map_err(|_| "The file path is not valid UTF-8.".to_owned())?;
+    let path = path.trim_end_matches(['\r', '\n']);
+    Ok((!path.is_empty()).then(|| path.to_owned()))
+}
+
 pub fn sibling(name: &str) -> PathBuf {
     std::env::current_exe()
         .ok()