Fix writing mode: prevent per-frame override and disable syntax highlighting

apply_syntax_to_buffer() was called every frame and unconditionally reset
edit mode from the file extension, making it impossible to toggle out of
writing mode for .txt/.md files. Add edit_mode_detected_ flag to Buffer so
auto-detection runs once per buffer. Writing mode now also disables syntax
highlighting as intended. Propagate edit_mode_ through Buffer copy/move ops.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Command.cc

@@ -1363,6 +1363,9 @@ cmd_toggle_edit_mode(const CommandContext &ctx)
 		b->ToggleEditMode();
 	}
 
+	// Writing mode disables syntax highlighting; code mode re-enables it.
+	b->SetSyntaxEnabled(b->GetEditMode() == EditMode::Code);
+
 	const char *mode_str = (b->GetEditMode() == EditMode::Writing) ? "writing" : "code";
 	ctx.editor.SetStatus(std::string("Mode: ") + mode_str);
 	return true;