Add 5 new features: edit snippets, clear scrollback, category filter, find in terminal, startup commands

1. Edit Snippets: Edit button to modify existing snippets
2. Clear Scrollback: Cmd+Shift+K to clear all terminal output
3. Category Filter: Dropdown to filter snippets by category
4. Find in Terminal: Cmd+F to search terminal output (built-in)
5. Startup Commands: Configure commands to run when opening new tabs

All features tested and working

TerminalSession.swift

@@ -179,6 +179,13 @@ class TerminalSession: NSObject, NSTextViewDelegate, NSMenuDelegate {
             // Wait a bit for shell to initialize, then show prompt
             DispatchQueue.main.asyncAfter(deadline: .now() + 0.3) { [weak self] in
                 self?.showPrompt()
+                
+                // Run startup commands
+                for command in TerminalSettings.shared.startupCommands {
+                    DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
+                        self?.executeCommand(command)
+                    }
+                }
             }
         } catch {
             appendText("Error starting shell: \(error)\n")