Add submitter confirmation email to WorkshopIdea

[maebeale]

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

• WorkshopIdea was the only submission model missing a submitter confirmation email
• All other submission models (StoryIdea, WorkshopVariationIdea, WorkshopLog, EventRegistration) already send both a submitter email and an admin FYI
• This aligns WorkshopIdea with the established pattern

How did you approach the change?

• Added idea_submitted notification call in WorkshopIdeasController#create, before the existing idea_submitted_fyi admin notification
• Uses the same idea_submitted kind already used by StoryIdea and WorkshopVariationIdea — the mailer method and templates already exist
• Added request spec verifying both notifications are sent on create (matching the pattern in workshop_variation_ideas_spec.rb)

UI Testing Checklist

• Create a new WorkshopIdea as a regular user — verify submitter receives confirmation email
• Verify admin still receives FYI notification email
• Verify email subject reads "AWBW Portal: Your workshop idea has been received"

Anything else to add?

• No new mailer methods or templates needed — the existing NotificationMailer#idea_submitted handles all idea types polymorphically via model_name.human

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