implement file open from Finder, save preserves format, save-as with format choice

src/AppDelegate.swift

@@ -43,6 +43,11 @@ class AppDelegate: NSObject, NSApplicationDelegate {
         )
     }
 
+    func application(_ application: NSApplication, open urls: [URL]) {
+        guard let url = urls.first else { return }
+        appState.loadNoteFromFile(url)
+    }
+
     func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
         return true
     }
@@ -169,7 +174,7 @@ class AppDelegate: NSObject, NSApplicationDelegate {
 
     @objc private func openNote() {
         let panel = NSOpenPanel()
-        panel.allowedContentTypes = [UTType(filenameExtension: "md")!, .plainText]
+        panel.allowedContentTypes = Self.supportedContentTypes
         panel.canChooseFiles = true
         panel.canChooseDirectories = false
         panel.allowsMultipleSelection = false
@@ -180,13 +185,21 @@ class AppDelegate: NSObject, NSApplicationDelegate {
     }
 
     @objc private func saveNote() {
-        appState.saveNote()
+        if appState.currentFileURL != nil {
+            appState.saveNote()
+        } else {
+            saveNoteAs()
+        }
     }
 
     @objc private func saveNoteAs() {
         let panel = NSSavePanel()
-        panel.allowedContentTypes = [UTType(filenameExtension: "md")!]
+        panel.allowedContentTypes = Self.supportedContentTypes
         panel.nameFieldStringValue = defaultFilename()
+        if let url = appState.currentFileURL {
+            panel.directoryURL = url.deletingLastPathComponent()
+            panel.nameFieldStringValue = url.lastPathComponent
+        }
         panel.beginSheetModal(for: window) { [weak self] response in
             guard response == .OK, let url = panel.url else { return }
             self?.appState.saveNoteToFile(url)
@@ -194,6 +207,9 @@ class AppDelegate: NSObject, NSApplicationDelegate {
     }
 
     private func defaultFilename() -> String {
+        if let url = appState.currentFileURL {
+            return url.lastPathComponent
+        }
         let firstLine = appState.documentText
             .components(separatedBy: "\n").first?
             .trimmingCharacters(in: .whitespaces) ?? ""
@@ -201,11 +217,74 @@ class AppDelegate: NSObject, NSApplicationDelegate {
             of: "^#+\\s*", with: "", options: .regularExpression
         )
         let trimmed = stripped.trimmingCharacters(in: .whitespaces)
-        guard !trimmed.isEmpty, trimmed != "Untitled" else { return "note.md" }
+        let ext = extensionForFormat(appState.currentFileFormat)
+        guard !trimmed.isEmpty, trimmed != "Untitled" else { return "note.\(ext)" }
         let sanitized = trimmed.map { "/:\\\\".contains($0) ? "-" : String($0) }.joined()
-        return sanitized.prefix(80) + ".md"
+        return sanitized.prefix(80) + ".\(ext)"
     }
 
+    private func extensionForFormat(_ format: FileFormat) -> String {
+        switch format {
+        case .markdown: return "md"
+        case .csv: return "csv"
+        case .json: return "json"
+        case .toml: return "toml"
+        case .yaml: return "yaml"
+        case .xml: return "xml"
+        case .svg: return "svg"
+        case .rust: return "rs"
+        case .c: return "c"
+        case .cpp: return "cpp"
+        case .objc: return "m"
+        case .javascript: return "js"
+        case .typescript: return "ts"
+        case .jsx: return "jsx"
+        case .tsx: return "tsx"
+        case .html: return "html"
+        case .css: return "css"
+        case .scss: return "scss"
+        case .less: return "less"
+        case .python: return "py"
+        case .go: return "go"
+        case .ruby: return "rb"
+        case .php: return "php"
+        case .lua: return "lua"
+        case .shell: return "sh"
+        case .java: return "java"
+        case .kotlin: return "kt"
+        case .swift: return "swift"
+        case .zig: return "zig"
+        case .sql: return "sql"
+        case .makefile: return "mk"
+        case .dockerfile: return "Dockerfile"
+        case .config: return "conf"
+        case .lock: return "lock"
+        case .plainText, .unknown: return "txt"
+        }
+    }
+
+    private static let supportedContentTypes: [UTType] = {
+        let extensions = [
+            "md", "markdown", "mdown",
+            "csv", "json", "toml", "yaml", "yml", "xml", "svg",
+            "rs", "c", "cpp", "cc", "cxx", "h", "hpp", "hxx",
+            "js", "jsx", "ts", "tsx",
+            "html", "htm", "css", "scss", "less",
+            "py", "go", "rb", "php", "lua",
+            "sh", "bash", "zsh", "fish",
+            "java", "kt", "kts", "swift", "zig", "sql",
+            "mk", "ini", "cfg", "conf", "env",
+            "lock", "txt", "text", "log"
+        ]
+        var types: [UTType] = [.plainText]
+        for ext in extensions {
+            if let t = UTType(filenameExtension: ext) {
+                types.append(t)
+            }
+        }
+        return Array(Set(types))
+    }()
+
     @objc private func openSettings() {
         SettingsWindowController.show()
     }