Audio Echo Effect Add echo and delay effects to audio files.

The echo effect uses a feedback delay network built with the Web Audio API. A DelayNode is configured with the user-specified delay time (0.1 to 2.0 seconds). The feedback loop is created by connecting the delay output through a GainNode (feedback amount, 0-0.9) back to the delay input. The wet/dry mix is implemented with two GainNodes: one for the dry (original) signal and one for the wet (delayed) signal, whose gains sum to 1.0. The complete graph is: source → dryGain → merger, and source → delay → feedbackGain → delay, delay → wetGain → merger. An OfflineAudioContext renders the full audio with sufficient extra length to capture the echo tail (duration + delay * repetitions). The output is encoded as WAV.