Product select dropdown

[Bouni]

Searchable product selection in the invoice editor

The product <select> on each invoice line is replaced by a combobox. You can type to filter products and pick one from the results, which helps once the product catalogue gets long.

What's new

• Live search: results filter as you type. The search covers name, SKU, description, category and unit.
• Multi-term matching: every word you type has to match, so web hour finds Web development billed per hour.
• More detail per result: each entry shows the product name, with SKU and price per unit underneath (e.g. WEB-01 · 95.00 / hour).
• Readable list in a narrow column: the dropdown is at least 18rem wide, even though the product column is narrow.
• Keyboard support: ↑ / ↓ to move, Enter to select, Esc to close, Tab to move on. When the list opens, the current product is highlighted and scrolled into view.
• Clear button: a selected product can be cleared with ✕.
• Accessibility: uses ARIA combobox/listbox roles, aria-activedescendant and aria-expanded.

Behaviour

• Choosing a product fills in description, unit price, unit and (in per-line tax mode) tax %, as before.
• Choosing the product that's already selected does not fill the line again, so manual edits to that line are kept.
• Clearing only removes the product link. Description, price and other fields on the line stay as they are.

Implementation

• New component frontend/src/lib/components/ProductCombobox.svelte (Svelte 5 runes, DaisyUI styling). It takes products, value and an onchange(productId) callback.
• InvoiceEditor.svelte uses the new component in place of the <select>. Each row gets its own ID (product-<itemId>), so ARIA references don't clash between lines.
• New i18n keys Clear and No products found are added to all locales (de, nl, es-ar, es-co, pt-br, tr).

Notes

• This PR works the same way as the customer combobox PR (customer-selection). Both add the Clear key and touch InvoiceEditor.svelte, so whichever merges second will have small conflicts that are easy to resolve.
• Once both are merged, the two components can be combined into one generic Combobox in a follow-up.

Testing

• bun run check: 0 errors
• eslint / prettier pass on the changed files
• Manual check: add a line, search for a product by name / SKU, select it, and confirm description, price, unit and tax are filled in
• Manual check: edit a line by hand, pick the same product again, and confirm the edits are kept
• Manual check: edit an existing invoice and confirm each line's product is preselected

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