P4: MIDI control surface mapping — faders/buttons over USB MIDI

C++ backend (fully implemented):
- MidiMapper: CC processing, mapping table, JSON persistence, learn mode
- MidiLearnMode: 3-step learn workflow state machine
- MixerEngine::processMidiEvent() wired into AudioHost MIDI pipeline
- MixerApi + PiPedalSocket: all WS handlers (getMidiMappings,
  setMidiLearnMode, setMidiLearnTarget, commitMidiLearn, etc.)

React frontend (new):
- MidiMappingPanel: dialog with learn mode toggle, CC capture polling,
  commit workflow, current mappings list with delete, manual add
- MixerPage: MIDI button in toolbar, learn mode state management
- ChannelStrip + MasterBus: learn mode callbacks on fader/mute/solo touch

src/MidiLearnMode.hpp

@@ -0,0 +1,66 @@
+// Copyright (c) 2026 Ourpad Network
+// See LICENSE file in the project root for full license text.
+
+#pragma once
+
+#include <cstdint>
+#include <mutex>
+#include "MidiMapper.hpp"
+
+namespace pipedal {
+
+/// MIDI Learn mode state machine.
+///
+/// Tracks the three-step learn workflow:
+///   1. User enables learn mode and touches a UI control (setPendingTarget)
+///   2. User moves a hardware fader — the CC event is captured (captureEvent)
+///   3. User confirms — a new MidiMappingEntry is created (commitMapping)
+///
+class MidiLearnMode {
+public:
+    MidiLearnMode();
+    ~MidiLearnMode();
+
+    /// Enable or disable learn mode.
+    void setEnabled(bool enabled);
+    bool isEnabled() const { return enabled_; }
+
+    /// Set the mixer parameter that should receive the next learned mapping.
+    /// Call this when the user touches a UI control while in learn mode.
+    void setPendingTarget(MidiTargetType type, int64_t id);
+
+    /// Get the current pending target.
+    bool getPendingTarget(MidiTargetType& outType, int64_t& outId) const;
+
+    /// Capture a MIDI CC event while in learn mode.
+    /// Call this from the RT audio thread when processEvent sees a CC.
+    void captureEvent(int midiChannel, int ccNumber);
+
+    /// Check if a CC event has been captured since learn mode was entered
+    /// or since the last clear().
+    bool hasCapturedEvent() const;
+
+    /// Get the last captured CC event info.
+    /// Returns true if an event was captured.
+    bool getCapturedEvent(int& outMidiChannel, int& outCcNumber) const;
+
+    /// Build a MidiMappingEntry from pending target + captured event.
+    /// Clears the captured event after building (avoids stale recomit).
+    MidiMappingEntry buildMapping() const;
+
+    /// Clear captured event without committing.
+    void clearCapturedEvent();
+
+    /// Reset all learn state.
+    void reset();
+
+private:
+    bool enabled_ = false;
+    MidiTargetType pendingTargetType_ = MidiTargetType::ChannelVolume;
+    int64_t pendingTargetId_ = 0;
+    int capturedMidiChannel_ = -1;
+    int capturedCcNumber_ = -1;
+    mutable std::mutex mutex_;
+};
+
+} // namespace pipedal