fix(preset-maker): validate presets before export

[Microck]

Summary

• adds visible multi-preset controls to the preset maker UI
• blocks invalid exports when preset IDs are missing or duplicated
• validates custom day values so out-of-range values do not generate broken JSON
• prevents invalid downloads from writing a config file

Verification

• verified locally through browser automation on the patched page
• confirmed single-preset generation still works
• confirmed multi-preset generation works from the new UI
• confirmed duplicate preset IDs are rejected
• confirmed invalid custom day values like 31 and -2 are rejected
• confirmed Copy still works
• confirmed Download JSON writes the expected file for valid input and does not write one for invalid input

Notes

• left unrelated untracked frontend.md out of the commit