fix(budget): fix invoice/work-item cross-linking

[steilerDev]

Summary

• InvoiceDetailPage: Replace the "Budget Line" row (which showed a raw UUID) with a "Work Item" row displaying the linked work item's title as a link to /work-items/:id. Uses invoice.workItemBudget already returned by the API — no additional fetch needed.
• WorkItemDetailPage: Invoice popover items now link to /budget/invoices/:id (invoice detail page) instead of /budget/vendors/:vendorId.

Test plan

• Open an invoice detail page for an invoice linked to a budget line — confirm "Work Item" row appears with the work item title as a clickable link, and clicking it navigates to the correct work item page
• Open an invoice detail page for an invoice with no budget line link — confirm the row is absent
• Open a work item detail page with budget lines that have invoices — open the invoice popover and click an invoice — confirm it navigates to /budget/invoices/:id

🤖 Generated with Claude Code

[ghost]

🎉 This PR is included in version 1.9.0-beta.49 🎉

The release is available on GitHub release

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