feat(gallery): add R26 response graph

A frequency-domain plot after a mastering processor's response display (references filed in the catalogue's working directory): log-frequency grid from 32 Hz to 16 kHz with labeled dB rails, an amber bell curve drawn live from a peak the user places, and a handle draggable in both axes at once. The kit's other traces are time-domain; this is its first x/y function display and its first two-axis drag.

I verified it in headless Chrome: press places the peak, dragging moves it in frequency and gain together, the curve and readout follow, no console exceptions.

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