Add toggle UX for workshop selection on story ideas

[Copilot]

What is the goal of this PR and why is this important?

Users need an intuitive way to enter custom workshop titles when submitting story ideas. Previously, both dropdown and text field were always visible, causing confusion about which to use.

How did you approach the change?

Frontend: Stimulus controller manages toggle between dropdown and text input

• "New Workshop" option triggers field swap
• X button clears and returns to dropdown
• State preserved when editing records with external titles

Backend: Controller prepends virtual "New Workshop" option, converts workshop_id: "new" to nil

Validation: Custom validator ensures either workshop_id or external_workshop_title present

Code organization: Extracted view conditionals to helpers (workshop_selected_value, show_external_workshop_field?)

# Controller adds virtual option
new_workshop_option = OpenStruct.new(id: "new", type_name: "New Workshop")
@workshops = [ new_workshop_option ] + Workshop.order(:title).to_a

# Params handler converts sentinel value
permitted_params[:workshop_id] = nil if permitted_params[:workshop_id] == "new"

Anything else to add?

Model tests confirm validation logic. Request spec verifies "new" value handling. CodeQL scan clean.

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