Milestone Widget

[wavehassman]

Changes

Adds an onboarding milestones timeline widget for new members, plus the admin tooling to manage what shows on it

Test Cases

• Onboarding dashboard widget shows only new-member-flagged milestones, sorted soonest-first, with past ones visually dimmed.
• Empty state when no milestones are flagged for either dashboard.
• "Add Onboarding Milestone" (Onboarding tab) creates a milestone visible there but not on the Recruitment tab; "Add Recruiting Milestone" (Recruitment tab) is the mirror case.
• Editing a milestone on either table (name/description/date) never moves it to the other table.
• Deleting a milestone from either table removes it everywhere (including the dashboard widget).

Screenshots

Checklist

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Closes #4121