feat(invoices): per-line-item KDV rates with mixed-rate support

[AltanEsmer]

Summary

• Adds line_items[] to the Invoice schema so a single invoice can carry products with different KDV rates (e.g. %10 and %16 on the same invoice — a Turkish tax requirement).
• Invoice-level subtotal / vat_amount / total become cached aggregates, always recomputed server-side from line_items. Invoice-level vat_rate is the single shared rate or null when mixed ("Karışık").
• New editable line-items table in the invoice form; manual total override retained only for single-line invoices.
• Invoice list shows a Karışık tag with a tooltip listing the distinct rates, and rows expand to reveal the per-line breakdown (with TRY equivalents for foreign-currency invoices).
• Excel export now includes a second Kalemler sheet with one row per line item; the main Faturalar sheet writes Karışık in the KDV Oranı column for mixed-rate invoices.
• Backward-compatible: an idempotent migration in loadData() synthesizes a single line from the existing flat fields for every legacy invoice on first load.

Files touched

• database.js — new _computeInvoiceTotals helper, line-items migration, payload normalization in addInvoice / updateInvoice, per-line TRY equivalents in _computeTryEquivalent, dashboard aggregation uses the cached vat_amount, updated sample seed.
• src/components/InvoiceFormLineItems.jsx (new) — editable line-items table with per-line chip-based KDV selector.
• src/components/InvoiceFormMoneyZone.jsx — trimmed to currency + manual-total switch + total display + FX reveal.
• src/pages/InvoiceForm.js — drives lineItems state, disables manual total for multi-line invoices.
• src/components/InvoiceTable.jsx — Karışık tag in the KDV column, expandable rows showing line items.
• src/pages/InvoiceList.js — builds the second-sheet lineItems payload; main sheet tolerates the Karışık sentinel.
• main.js — export-to-excel handler optionally writes the Kalemler sheet and writes non-numeric KDV cells as text.
• src/index.css — styles for the line-items table and summary strip.
• docs/DATA-MODEL.md — documents the new line_items field and adds a mixed-rate invoice example.

Test plan

• npm run dev — launch app, verify legacy invoices auto-migrated (no console errors, expanded row shows a single auto-generated line)
• Add a TRY invoice with two lines at %10 and %16 — list shows Karışık tag with tooltip; expanded row shows both lines; aggregates correct
• Edit a legacy invoice — single line prefilled, save round-trips with identical totals
• Add a USD invoice with two mixed-rate lines + an existing FX rate for that month — expanded row shows per-line TRY equivalents
• Edit the FX rate — confirm invoice-level and per-line TRY values both update
• Single-line invoice with manual total override — value persists; toggling to multi-line disables the override
• Export to Excel — Faturalar sheet shows Karışık in KDV Oranı for mixed rows; Kalemler sheet lists one row per line with line totals
• Dashboard VAT-by-month unchanged for legacy data; new mixed invoice contributes its full vat_amount

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