[1.8] Main app: invoice form E-Invoicing routing code fields (UI + validation)

[nielsdrost7]

Target repository: InvoicePlane (main app) · branch prep/v180
Related feature issue: #15
Reference doc: #43 — routing codes

What to do

Add a collapsible E-Invoicing section to the invoice create/edit form containing three text fields:

Field label	Meta key	Standard usage
Routing code 1	invoice_routing_code_1	Leitweg-ID (XRechnung) · CIG (FatturaPA B2G) · DIR3 managing body (Facturae)
Routing code 2	invoice_routing_code_2	CUP (FatturaPA B2G) · DIR3 processing unit (Facturae)
Routing code 3	invoice_routing_code_3	DIR3 accounting office (Facturae)

Labels are intentionally generic ("Routing code 1/2/3"). Tooltips explain the per-standard usage.

Show this section automatically when the selected client has an e-invoice template assigned.

Validation rules at save time

All three fields are optional at save. This allows saving an incomplete draft before the procurement routing codes are known (e.g. before the purchase order is received). XRechnung's mandatory Leitweg-ID check is enforced at generation time by the generator, not at save time.

Rule	Detail
Required at save	No
Format	Freetext
Max length	100 chars each

Acceptance criteria

• Routing code section visible on invoice create and edit forms when client uses e-invoicing
• Section hidden / collapsed for clients without an e-invoice template assigned
• All three fields accept freetext up to 100 chars with no format error
• No validation error when all three fields are submitted empty
• Tooltips present explaining per-standard usage of each code