feat: multi-step campaign creation form

[josephchimebuka]

Summary

Implements the Multi-Step Form Structure for campaign creation.

Closes #86

What's included

• Adds src/pages/campaigns/CreateCampaignPage.jsx as the multi-step form container.
• Manages the current step via formStep in the campaigns Redux slice (nextStep / prevStep / setFormStep).
• Renders the correct step component based on formStep: Details → Funding → Media → Review.
• Adds Next and Back buttons for navigation, plus a step progress indicator.
• Persists form data in draftCampaign Redux state as the user moves between steps, so data is retained when going back.
• Prompts an "unsaved changes" warning when leaving the page using useBeforeUnload.
• Routes /create (and /campaigns/create) to the new page, replacing the previous static placeholder.

Changes

• src/features/campaigns/campaignsSlice.js — add formStep + draftCampaign state with step/draft reducers and selectors (existing items/loading/error left intact)
• src/pages/campaigns/CreateCampaignPage.jsx — step container with navigation + useBeforeUnload
• src/pages/campaigns/steps/* — Details, Funding, Media, and Review step components
• src/routes/AppRouter.jsx — point the create route to the new page
• src/pages/CreateCampaign.jsx — removed (replaced by the multi-step page)

Acceptance criteria

• Users can navigate between steps with Next/Back
• Data persists when going back to a previous step (draftCampaign in Redux)
• Unsaved changes trigger a leave-page warning (useBeforeUnload)
• npm run build passes

Test plan

• Go to /create, fill in step 1, click Next, then Back — data is retained
• Refresh/close the tab with unsaved data — browser shows the warning
• Complete all steps to the Review screen and submit

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