List the shipped design templates

Scope: read. The same starting points the editor offers on first run: each template's id, its sheet names, a component census by node type, and the explanatory notes it carries on canvas. Start here when the user describes a design that probably already exists (an FPGA rail set, a USB-C PD supply, a motor driver). Static files shipped with the build; Cache-Control: private, max-age=3600.