Polish G-Code Visualization and download UI

- Fix intermittent bug where the Upload G-Code box stayed hidden after
certain source switches: toggle the box client-side instead of via a
racy server visibility update.
- Make all three G-code download boxes download-only (Shape 2 and 3 were
accidentally upload-enabled).
- Keep the download boxes the same compact height before and after
Generate by shrinking the empty-state placeholder.
- Reword the "all moves G1" toggle in plain language with a G0/G1
explanation in helper text.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app.py

@@ -149,6 +149,25 @@ APP_CSS = """
 .gcode-view .cm-scroller {
     overflow: auto;
 }
+
+/* Shrink the empty-state placeholder of the G-code download boxes so they are
+   the same compact height before and after Generate G-Code is pressed. */
+.gcode-download .empty {
+    min-height: 0 !important;
+    height: 35px !important;
+    padding: 0 !important;
+    overflow: hidden !important;
+}
+.gcode-download .empty svg {
+    width: 22px !important;
+    height: 22px !important;
+}
+
+/* The G-code upload box starts hidden; it is shown client-side only when
+   "Upload G-Code file" is selected (see gcode_source.change). */
+#gcode-upload-col {
+    display: none;
+}
 """
 
 # Gradio 6.10's gr.Model3D leaves the Undo (reset view) button permanently
@@ -1043,10 +1062,6 @@ GCODE_SOURCE_SHAPE3 = "Use Shape 3 G-Code"
 GCODE_SOURCE_UPLOAD = "Upload G-Code file"
 
 
-def toggle_gcode_source(source: str) -> dict[str, Any]:
-    return gr.update(visible=(source == GCODE_SOURCE_UPLOAD))
-
-
 def render_toolpath(
     source: str,
     uploaded_path: str | None,
@@ -1625,15 +1640,21 @@ def build_demo() -> gr.Blocks:
                 value=True,
             )
             gcode_all_g1 = gr.Checkbox(
-                label="Use G1 for all moves (no G0 travel commands) — applies to all shapes",
+                label="Move at one constant speed (no fast travel moves)",
+                info=(
+                    "Every move — including repositioning between deposits — uses the "
+                    "slower printing-speed command (G1) instead of a rapid travel command "
+                    "(G0). The valve still controls where material is actually dispensed. "
+                    "Applies to all shapes."
+                ),
                 value=True,
             )
             gcode_button = gr.Button("Generate G-Code", variant="primary")
 
             with gr.Row():
-                gcode_file_1 = gr.File(label="Download G-Code Shape 1", interactive=False)
-                gcode_file_2 = gr.File(label="Download G-Code Shape 2")
-                gcode_file_3 = gr.File(label="Download G-Code Shape 3")
+                gcode_file_1 = gr.File(label="Download G-Code Shape 1", interactive=False, elem_classes=["gcode-download"])
+                gcode_file_2 = gr.File(label="Download G-Code Shape 2", interactive=False, elem_classes=["gcode-download"])
+                gcode_file_3 = gr.File(label="Download G-Code Shape 3", interactive=False, elem_classes=["gcode-download"])
 
             # Per-shape G-code preview, aligned under each download box. Fixed
             # height with internal scrolling so the files don't fill the page.
@@ -1730,10 +1751,11 @@ def build_demo() -> gr.Blocks:
                     value=GCODE_SOURCE_SHAPE1,
                     label="G-Code source",
                 )
-                # Visibility is toggled on this wrapper column rather than the
-                # File itself: a File that is the output of the toggle event
-                # gets stuck showing a progress overlay when first revealed.
-                with gr.Column(visible=False) as upload_col:
+                # Shown only when "Upload G-Code file" is selected. Visibility is
+                # toggled client-side (see gcode_source.change below) instead of
+                # via a server round-trip, which raced and intermittently left
+                # the box hidden. Hidden initially by CSS (#gcode-upload-col).
+                with gr.Column(elem_id="gcode-upload-col") as upload_col:
                     gcode_upload = gr.File(
                         label="Upload G-Code",
                         file_types=[".txt", ".gcode", ".nc"],
@@ -1815,10 +1837,16 @@ def build_demo() -> gr.Blocks:
             render_mode = gr.State("tube")
 
             gcode_source.change(
-                fn=toggle_gcode_source,
+                fn=None,
                 inputs=[gcode_source],
-                outputs=[upload_col],
-                queue=False,
+                outputs=[],
+                js="""(src) => {
+                    const col = document.getElementById('gcode-upload-col');
+                    if (col) col.style.display = (src === '"""
+                + GCODE_SOURCE_UPLOAD
+                + """') ? 'flex' : 'none';
+                    return [];
+                }""",
             )
             render_inputs = [gcode_source, gcode_upload, gcode_file_1, gcode_file_2, gcode_file_3, travel_opacity_slider, print_opacity_slider, travel_color_picker, print_color_picker, print_width_slider, travel_width_slider]
             render_line_button.click(