Fix audio preview (AudioBuffer redesign) and insert-to-DAW drag initiation

[Copilot]

Two broken features in the Library tab: audio preview produced silence, and "Insert into DAW" created file copies instead of initiating a DAW drag.

Audio Preview — PluginPreview redesign

AudioTransportSource was silently failing: setSource() called from the message thread requires precise prepareToPlay() ordering and a proper buffering/thread strategy that wasn't in place.

Replaced with a straightforward AudioBuffer<float>-backed implementation:

• loadFile() decodes the entire file into memory; linear-interpolation resampling applied at load time when source ≠ DAW sample rate
• play() / stop() flip std::atomic<int> playPos_ — no locks
• render() copies from the in-memory buffer using the atomic position; ScopedTryLock keeps the audio thread wait-free during concurrent loadFile() calls
• Public API unchanged — PluginProcessor.cpp untouched

Insert to DAW — drag from mouseDrag, not onClick

performExternalDragDropOfFiles() requires an active mouse press; calling it from onClick (mouse-up) is a no-op on macOS, causing the fallback to reveal the file in Finder — which DAWs then import as a copy.

Added InsertDawDragListener (a juce::MouseListener inner struct on insertDawButton_):

void InsertDawDragListener::mouseDrag(const juce::MouseEvent& e)
{
    if (dragStarted_ || e.getDistanceFromDragStart() < 8) return;
    dragStarted_ = true;
    editor_.startInsertDawDrag(); // calls performExternalDragDropOfFiles()
}

A plain click (no drag) still falls back to revealing the file and copying the path to the clipboard.

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