feat: bulk ops, advanced sort, creator messaging, duplicate detection

[ice-man123]

Summary

This PR implements four features from the backlog, closing issues #810, #811, #812, and #813.

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#810 — Bulk invoice operations (export, print, archive)

New: src/lib/bulkInvoiceOps.ts

• exportInvoicesBulk(invoices, format) — downloads selected invoices as CSV or JSON
• printInvoicesBulk(invoices) — opens a formatted, print-ready window with a Print / Save as PDF button
• archiveInvoicesBulk / unarchiveInvoicesBulk — wrappers over the existing archive lib

Updated: src/components/BulkActionBar.tsx

• Added a Print button
• Replaced Export CSV with an Export ▾ dropdown offering CSV and JSON
• All existing archive / unarchive / cancel / done actions preserved

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#811 — Advanced sort options for invoice lists

Updated: src/lib/dashboardFilters.ts

• 7 new DashboardSortId values: deadline-farthest, funded-pct-desc/asc, recipients-desc/asc, creator-asc/desc
• SORT_OPTIONS entries carry a group tag; exported SORT_GROUPS for grouped rendering

New: src/components/invoices/AdvancedSortSelect.tsx

• Grouped <select> with <optgroup> sections: Date, Deadline, Amount, Funding, Recipients, Creator

Updated: src/components/DashboardClient.tsx

• Uses AdvancedSortSelect in place of the previous plain <select>

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#812 — Creator messaging system for payer communication

New: src/lib/creatorMessages.ts

• Full messaging API: postMessage, loadMessages, markThreadRead, countUnread, deleteMessage, resolveRole, formatMessageTime
• localStorage-backed per-invoice threads (same pattern as InvoiceChat)

New: src/components/CreatorMessagePanel.tsx

• Collapsible panel with unread badge, role labels (Creator/Payer/System), delete-own-message on hover, and clear permission messaging for non-participants

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#813 — Invoice duplicate detection and merge

New: src/lib/invoiceDuplicates.ts

• Exact and fuzzy duplicate detection via invoiceFingerprint
• mergeInvoiceGroup(group, strategy) — hides duplicates in localStorage; strategies: keep-newest, keep-oldest, keep-largest
• undoMerge, filterMerged, describeDifferences

New: src/components/DuplicateMergeModal.tsx

• Lists duplicate groups by confidence (Exact / Fuzzy)
• Per-group diff summary, strategy selector, and undo support

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Checklist

• Feature implemented for all four issues
• Mobile-responsive (flex-wrap, max-h / overflow-y-auto)
• Accessible (aria labels, roles, aria-expanded, aria-live)
• TypeScript — all public APIs typed, no new any casts
• No changes to tasks.md committed

Closes #810
Closes #811
Closes #812
Closes #813

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