Age ranges: single primary like sectors + visibility toggle

[maebeale]

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

• Builds on Populate Additional Ages Served from age range categories #1717's primary/additional age groups, reshaping them to match sectors exactly and adding a per-profile visibility toggle.
• On the registration form a respondent now picks one primary age range from a dropdown and many additional ones from a multi-select — same shape as the primary/additional sector questions.

How did you approach the change?

Registration form

• "Primary age range" is now a single-select dropdown (one primary); "Additional ages served" stays multi-select.
• A data migration converts existing forms' primary_age_group field to a dropdown and renames both questions.

Single primary (sector parity)

• AgeGroupTaggable#tag_age_groups and #apply_primary_age_groups! keep exactly one primary — choosing a new primary demotes the previous one — backed by an at_most_one_primary_age_group validation.
• A migration normalizes any existing data that marked several primaries down to one.
• Edit forms keep a single "Primary" toggle checked at a time via a new single-select-checkbox Stimulus controller (allows zero or one, like the star toggle on sectors).

Visibility toggle

• profile_show_age_ranges on people + organizations gates the "Age ranges served" section, mirroring profile_show_sectors. Added a "Show age ranges" checkbox to both edit forms.

Reporting

• The event dashboard age breakdown now reads both the primary and additional age-range answers (previously only the primary question), so the chart reflects every age range served.

Anything else to add?

• This branch was previously a parallel implementation of Populate Additional Ages Served from age range categories #1717; it has been rebased onto main and reduced to just the deltas above (single-primary + visibility toggle + dashboard fix). Everything else from the old version was superseded by Populate Additional Ages Served from age range categories #1717.
• Specs: updated AgeGroupTaggable spec (single-primary, demote, validation), added a builder spec for the new age field types. Full affected suite green; rubocop clean; dev seed + Vite dev build run.

🤖 Generated with Claude Code

[maebeale]

🤖 From Claude: Rebased onto latest main (33 commits). Conflicts resolved: form_builder_service.rb (main renamed hint: → subtitle: in add_field, so the new age-range fields use subtitle:), db/schema.rb version line, and the AGENTS.md controller count.

Heads-up on the backfill migration (20260617150000): it's backdated relative to main's hint_text → subtitle rename (20260618014554), so it runs before the rename on a fresh in-order DB but after it on production (where the rename is already applied). It detects column_exists?(:form_fields, :subtitle) and writes to whichever column exists, so the descriptive text lands correctly in both orderings. On a fresh DB there are no forms yet, so the loop is a no-op anyway.