Create Dashboard Slice

[grantfox-oss]

Description:
Create the Redux slice for dashboard-level data.

Tasks:

• Create src/features/dashboard/dashboardSlice.js
• State shape: { stats: null, recentDonations: [], recentCampaigns: [], isLoading: false, error: null }
• Async thunks: fetchDashboardStats, fetchRecentDonations, fetchRecentCampaigns
• Export selectors for each state field

Acceptance Criteria:
Thunks fetch data correctly; loading and error states update as expected.

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