Make automatic Tone Center read as automatic in examples

Switch the Tone Center control and example data away from a misleading `0 Hz` placeholder so automatic session shaping reads correctly in the UI. This keeps the examples aligned with the actual behavior: blank means Rhythma chooses the tone center from the session profile.

Constraint: `0` is not a meaningful tone center value in this product
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep automatic controls visually distinct from real numeric overrides in future UI changes
Tested: python -m py_compile app.py tests/test_rhythma_ui_copy.py
Tested: PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python -c "import pytest; raise SystemExit(pytest.main(['tests/test_rhythma_ui_copy.py','-q','-p','no:cacheprovider']))"
Not-tested: Live Gradio rendering in this environment because gradio is not installed locally

app.py

@@ -133,6 +133,17 @@ def normalize_rhythm_override(value):
     return value
 
 
+SESSION_EXAMPLES = [
+    ["I need something steady before a difficult conversation.", None, None, "sine", "Automatic", 12],
+    ["I want to feel grounded and open as the evening slows down.", None, None, "sine", "Automatic", 18],
+    ["I need a clear horizon for deep work.", None, None, "sine", "focused", 20],
+    ["Everything feels loud and I want a softer landing.", None, None, "sine", "Automatic", 14],
+    ["I feel bright and want a livelier pulse without losing calm.", None, None, "pulse", "active", 10],
+    ["Give me a long unwind after a heavy day.", None, None, "sine", "relaxed", 30],
+    ["I want a gentle session for a low-energy morning.", None, None, "sine", "Automatic", 16],
+]
+
+
 def build_runtime_session_profile(
     analysis_result,
     base_freq=None,
@@ -255,10 +266,13 @@ def create_interface():
                 )
 
                 with gr.Accordion("Session shaping controls", open=False):
-                    override_freq = gr.Slider(
-                        minimum=0, maximum=1000, value=0, step=1,
+                    override_freq = gr.Number(
+                        value=None,
+                        minimum=0,
+                        precision=0,
                         label="Tone Center (Hz)",
-                        info="Leave at 0 to let Rhythma choose a tone center from your session profile."
+                        placeholder="Automatic",
+                        info="Leave blank to let Rhythma choose a tone center from your session profile."
                     )
                     override_modulation = gr.Dropdown(
                         choices=["sine", "pulse", "chirp"],
@@ -313,15 +327,7 @@ def create_interface():
         )
 
         gr.Examples(
-            examples=[
-                ["I need something steady before a difficult conversation.", None, 0, "sine", "Automatic", 12],
-                ["I want to feel grounded and open as the evening slows down.", None, 0, "sine", "Automatic", 18],
-                ["I need a clear horizon for deep work.", None, 0, "sine", "focused", 20],
-                ["Everything feels loud and I want a softer landing.", None, 0, "sine", "Automatic", 14],
-                ["I feel bright and want a livelier pulse without losing calm.", None, 0, "pulse", "active", 10],
-                ["Give me a long unwind after a heavy day.", None, 0, "sine", "relaxed", 30],
-                ["I want a gentle session for a low-energy morning.", None, 0, "sine", "Automatic", 16],
-            ],
+            examples=SESSION_EXAMPLES,
             inputs=[input_text, audio_input, override_freq, override_modulation, override_rhythm, duration],
             outputs=[
                 session_name_output, emotional_tone_output, listening_path_output,

tests/test_rhythma_ui_copy.py

@@ -32,6 +32,12 @@ def test_normalize_rhythm_override_treats_automatic_as_none():
     assert app.normalize_rhythm_override("Automatic") is None
 
 
+def test_session_examples_leave_tone_center_blank_for_automatic_mode():
+    app = import_app_with_gradio_stub()
+
+    assert all(example[2] is None for example in app.SESSION_EXAMPLES)
+
+
 def test_build_session_copy_uses_human_labels():
     app = import_app_with_gradio_stub()
     analysis = {