Allow Unit to be set to empty

[Bouni]

Allow products without a unit

Closes #154

Problem

A product's unit couldn't be cleared:

• The backend replaced an empty unit with "piece" when creating, updating and reading products.
• The edit form sent undefined when the unit was cleared, so the backend kept the old value.

Changes

• backend/src/controllers/products.ts: removed the "piece" fallback. An empty unit is stored as NULL and returned as undefined.
• frontend/src/routes/products/[id]/edit/+page.server.ts: the form now sends the empty unit so the backend clears it.
• frontend/src/lib/components/InvoiceEditor.svelte: picking a product with no unit for an invoice line now clears that line's unit instead of keeping the previous one.

Behaviour change

New products with no unit selected now have no unit; they no longer default to piece. Existing products are not changed.

Testing

• Clear the unit of an existing product → it's saved as empty and shown as -
• Create a product without a unit → no unit is stored
• Add a product without a unit to an invoice → the line's unit is empty and the PDF has no unit column (unless another line has a unit)

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