Implement `get_all_milestones` view function

[ayshadogo]

User Story:
As a developer, I want to read all milestones in one call so that the frontend can display the full milestone timeline efficiently.
Acceptance Criteria:

• fn get_all_milestones(env: Env) -> Vec<MilestoneData>
• Returns milestones in index order
• No auth required
• Handles campaigns with 1–5 milestones

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