feat: add microui-based GUI with transport controls and progress bar

Co-authored-by: aider (deepseek/deepseek-coder) <aider@aider.chat>

engine.c

@@ -510,3 +510,114 @@ const char* quantize_mode_to_string(QuantizeMode mode) {
         default: return "Unknown";
     }
 }
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <math.h>
+#include <jack/jack.h>
+#include "engine.h"
+
+static float *audio_buffer = NULL;
+static int buffer_size = 0;
+static float bpm = 120.0f;
+static int loop_length = 8;
+static int current_beat = 0;
+static int active = 0;
+static jack_nframes_t sample_rate = 0;
+static jack_port_t *output_port = NULL;
+
+static int process_callback(jack_nframes_t nframes, void *arg)
+{
+    (void)arg;
+    jack_default_audio_sample_t *out = jack_port_get_buffer(output_port, nframes);
+    if (!out) return 0;
+
+    /* simple metronome: generate a click on each beat */
+    float samples_per_beat = sample_rate * 60.0f / bpm;
+    static float phase = 0.0f;
+
+    for (jack_nframes_t i = 0; i < nframes; i++) {
+        if (phase >= samples_per_beat) {
+            phase -= samples_per_beat;
+            current_beat = (current_beat + 1) % loop_length;
+        }
+
+        float sample = 0.0f;
+        if (active) {
+            /* short click at start of beat */
+            float click_duration = samples_per_beat * 0.05f;
+            if (phase < click_duration) {
+                float t = phase / click_duration;
+                sample = sinf(t * M_PI) * 0.3f;
+            }
+        }
+        out[i] = sample;
+        phase += 1.0f;
+    }
+
+    return 0;
+}
+
+int engine_init(jack_client_t *client, float *buffer, int buf_size)
+{
+    audio_buffer = buffer;
+    buffer_size = buf_size;
+
+    sample_rate = jack_get_sample_rate(client);
+
+    output_port = jack_port_register(client, "output",
+                                     JACK_DEFAULT_AUDIO_TYPE,
+                                     JackPortIsOutput, 0);
+    if (!output_port) {
+        fprintf(stderr, "no more JACK ports available\n");
+        return -1;
+    }
+
+    jack_set_process_callback(client, process_callback, NULL);
+
+    if (jack_activate(client)) {
+        fprintf(stderr, "cannot activate client\n");
+        return -1;
+    }
+
+    return 0;
+}
+
+void engine_cleanup(jack_client_t *client)
+{
+    jack_deactivate(client);
+    jack_port_unregister(client, output_port);
+}
+
+int engine_start(jack_client_t *client)
+{
+    (void)client;
+    active = 1;
+    return 0;
+}
+
+int engine_stop(jack_client_t *client)
+{
+    (void)client;
+    active = 0;
+    return 0;
+}
+
+void engine_set_bpm(jack_client_t *client, float new_bpm)
+{
+    (void)client;
+    bpm = new_bpm;
+}
+
+void engine_set_loop_length(jack_client_t *client, int beats)
+{
+    (void)client;
+    loop_length = beats;
+    if (current_beat >= loop_length) current_beat = 0;
+}
+
+int engine_get_current_beat(jack_client_t *client)
+{
+    (void)client;
+    return current_beat;
+}