feat: Top investors leaderboard, edit invoice page, admin review page, and optimistic notification updates

[makuo567]

Summary

This PR addresses and resolves issues #53, #52, #41, and #42 in StellarState/SS-frontend.

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Features & Fixes Included

1. Top Investors Leaderboard Section (#53)

• Component & Hook: Implemented TopInvestorsLeaderboard component and useLeaderboard hook fetching from /api/leaderboard.
• Refetch Interval: Configured to auto-refresh every 5 minutes (refetchInterval: 300,000ms).
• Data Rendering: Renders rank, truncated wallet address (truncateAddress), total committed XLM (formatXLM), and invoice count in descending order of total committed XLM (top 5).
• Loading & Empty States: Displays skeleton rows while loading and automatically hides the section if fewer than 1 investor is returned.
• Home Page: Integrated into app/page.tsx.

2. Edit Draft Invoice Page for Sellers (#52)

• Component & Route: Created EditInvoiceForm component and routes at /seller/edit/[id] and /seller/invoices/[id]/edit.
• Pre-populated Fields: Pre-populates title, description, face value, and funding deadline from existing invoice data.
• Document Replacement: Displays existing document CID with a "Replace document" option integrating DocumentUpload to upload to IPFS.
• Save & Navigation: Sends a PATCH request on save with updated fields and redirects back to the seller dashboard (/seller).
• Access Guard: Blocks non-draft invoices with a "This invoice cannot be edited" message and hides editing form controls.

3. Admin Invoice Review Page (#41)

• Component & Route: Created AdminInvoiceReview component and routes at /admin/review, /admin/invoices, and /admin.
• Pending List: Fetches and lists all pending_review invoices displaying title, seller wallet (truncated), face value, and submission date.
• Approve Action: Transitions status to published and removes the invoice from the pending list.
• Reject Action: Requires an inline rejection reason input before confirming rejection.
• Non-Admin Guard: Redirects non-admin wallets to the home page with a "Not authorised" toast.
• Empty State: Renders clear empty state when no invoices are pending review.

4. Optimistic Update for Notification Read Action (#42)

• Hook Optimization: Updated useMarkNotificationRead using React Query's onMutate, onError, and onSettled.
• Optimistic UI: Immediately clears the unread indicator on click and decrements the navbar bell unread counter before server response.
• Rollback & Invalidation: Cancels in-flight queries on mutate, restores unread state and nav bell count on error/rollback, and invalidates notification list cache when settled.
• Navbar Bell: Added notification bell with unread badge counter in Navbar.tsx.

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Verification & Testing

• Unit tests written and passing for all four features (TopInvestorsLeaderboard.test.tsx, EditInvoiceForm.test.tsx, AdminInvoiceReview.test.tsx, useNotifications.test.tsx).
• Verified TypeScript compilation (npm run type-check) passed with zero errors.
• Verified Next.js build (npm run build) succeeded cleanly.

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closes #53
closes #52
closes #41
closes #42

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