feat: support double-clicking .db/.sqlite/.duckdb files to open in IDE

Add OS file association for database files (.db, .sqlite, .sqlite3, .duckdb)
so double-clicking them in Finder/Explorer automatically creates a connection.
If the same file path is already connected, prompts the user to switch instead.

Also fix 42 pre-existing clippy warnings in dbx-core (needless_borrow,
too_many_arguments, derivable_impls, manual_div_ceil, type_complexity, etc.).

Closes #527

src-tauri/src/commands/external_db.rs

@@ -0,0 +1,113 @@
+use std::path::{Path, PathBuf};
+use std::sync::Mutex;
+
+const DB_EXTENSIONS: &[&str] = &["db", "sqlite", "sqlite3", "duckdb"];
+
+#[derive(Default)]
+pub struct ExternalDbOpenState {
+    pending: Mutex<Vec<String>>,
+}
+
+impl ExternalDbOpenState {
+    pub fn push(&self, paths: Vec<String>) {
+        if paths.is_empty() {
+            return;
+        }
+        if let Ok(mut pending) = self.pending.lock() {
+            pending.extend(paths);
+        }
+    }
+
+    fn drain(&self) -> Vec<String> {
+        self.pending.lock().map(|mut pending| pending.drain(..).collect()).unwrap_or_default()
+    }
+}
+
+#[tauri::command]
+pub fn pending_open_db_files(state: tauri::State<'_, ExternalDbOpenState>) -> Vec<String> {
+    let cwd = std::env::current_dir().unwrap_or_else(|_| PathBuf::from("."));
+    let mut paths = db_file_paths_from_args(std::env::args().skip(1), &cwd);
+    paths.extend(state.drain());
+    dedupe_paths(paths)
+}
+
+pub fn db_file_paths_from_args<I, S>(args: I, cwd: &Path) -> Vec<String>
+where
+    I: IntoIterator<Item = S>,
+    S: AsRef<str>,
+{
+    args.into_iter().filter_map(|arg| db_file_path_from_arg(arg.as_ref(), cwd)).collect()
+}
+
+fn db_file_path_from_arg(arg: &str, cwd: &Path) -> Option<String> {
+    if arg.starts_with('-') {
+        return None;
+    }
+
+    let path = PathBuf::from(arg);
+    if !is_db_file_path(&path) {
+        return None;
+    }
+
+    let resolved = if path.is_absolute() { path } else { cwd.join(path) };
+    Some(resolved.to_string_lossy().to_string())
+}
+
+pub fn is_db_file_path(path: &Path) -> bool {
+    path.extension()
+        .and_then(|ext| ext.to_str())
+        .map(|ext| DB_EXTENSIONS.iter().any(|e| e.eq_ignore_ascii_case(ext)))
+        .unwrap_or(false)
+}
+
+fn dedupe_paths(paths: Vec<String>) -> Vec<String> {
+    let mut unique = Vec::new();
+    for path in paths {
+        if !unique.contains(&path) {
+            unique.push(path);
+        }
+    }
+    unique
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn filters_db_file_args_case_insensitively() {
+        let paths = db_file_paths_from_args(
+            ["/tmp/a.db", "--flag", "/tmp/b.SQLITE", "/tmp/c.sqlite3", "/tmp/d.duckdb", "/tmp/e.txt"],
+            Path::new("/work"),
+        );
+
+        assert_eq!(paths, vec!["/tmp/a.db", "/tmp/b.SQLITE", "/tmp/c.sqlite3", "/tmp/d.duckdb"]);
+    }
+
+    #[test]
+    fn resolves_relative_db_file_args_against_cwd() {
+        let paths = db_file_paths_from_args(["data/mydb.db"], Path::new("/work"));
+
+        assert_eq!(paths, vec!["/work/data/mydb.db"]);
+    }
+
+    #[test]
+    fn drains_pending_db_file_paths_once() {
+        let state = ExternalDbOpenState::default();
+        state.push(vec!["/tmp/a.db".to_string()]);
+
+        assert_eq!(state.drain(), vec!["/tmp/a.db"]);
+        assert!(state.drain().is_empty());
+    }
+
+    #[test]
+    fn is_db_file_path_recognizes_extensions() {
+        assert!(is_db_file_path(Path::new("test.db")));
+        assert!(is_db_file_path(Path::new("test.sqlite")));
+        assert!(is_db_file_path(Path::new("test.sqlite3")));
+        assert!(is_db_file_path(Path::new("test.duckdb")));
+        assert!(is_db_file_path(Path::new("test.DB")));
+        assert!(!is_db_file_path(Path::new("test.sql")));
+        assert!(!is_db_file_path(Path::new("test.txt")));
+    }
+}

src-tauri/src/commands/mod.rs

@@ -9,6 +9,7 @@ pub mod csv_export;
 pub mod data_compare;
 pub mod database_export;
 pub mod deep_link;
+pub mod external_db;
 pub mod external_sql;
 pub mod history;
 pub mod mcp_bridge;