feat(export): OpenGD77 radio profile selection at export time

[pskillen]

Problem

#43 documents OpenGD77 as a two-layer reference: generic wire format + per-radio profiles (docs/reference/opengd77/radios/). The internal codeplug model is radio-agnostic; limits (zone member count, TG list member count, max channels) should be applied when the operator picks a target radio at export.

Today's exporter hard-codes Baofeng 1701 constants (zoneMemberHeaders(80), rxGroupListMemberHeaders(32)) with no profile picker on /#/export.

Intended outcome

• Export UI: pick OpenGD77 radio profile (1701 first; GD-77 / MD-UV380 stubs later).
• Serialiser reads profile limits when padding/truncating member columns and validating counts.
• Profile constants move out of hard-coded columns.ts defaults into profile config keyed by slug.

References

• docs/reference/opengd77/radios/README.md
• docs/reference/opengd77/radios/baofeng-1701.md
• Discovered during docs: authoritative OpenGD77 CPS CSV field reference under docs/reference/opengd77/ #43