fix: add empty states for transactions and bills views (closes #175)

[Emmanard]

Summary

Wires the existing EmptyStateIllustration component (which was built but unused) into all missing and partial empty state slots across the app.

Changes

• category-grid.tsx — replaces raw inline SVG with EmptyStateIllustration variant="search"
• recent-billers.tsx — replaces invisible return null on search no-results with a proper empty state card
• portfolio-page-client.tsx — replaces bare text with EmptyStateIllustration variant="empty" + Add Funds CTA
• recent-transactions.tsx — adds defensive empty state guard for when transactions array is empty
• transaction-history.tsx — adds empty state with Clear Filters CTA, hides table/pagination when no results

Approach

• Zero new libraries or icon sets introduced
• All changes are additive only — no existing logic altered
• Follows the exact pattern established in scheduled-payments.tsx and saved-accounts.tsx
• clearFilters() helper resets only pre-existing state setters to their original initial values

Closes #175

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