[1.8] Main app: client form E-Invoicing fieldset (UI + validation)

[nielsdrost7]

Target repository: InvoicePlane (main app) · branch prep/v180
Related feature issues: #13 · #14
Reference docs: #41 — client_peppol_id · #42 — client_company_number

What to do

Add an E-Invoicing section (collapsible fieldset or separate tab) to the client create/edit form:

Field label	DB field / meta key	Storage
Peppol participant ID	client_peppol_id	Column on ip_clients
SdI routing code	client_einvoice_sdi_code	Key in ip_client_meta
PEC email address	client_einvoice_pec_email	Key in ip_client_meta
Company registration number	client_company_number	Column on ip_clients

Validation rules

Field	Rule
client_peppol_id	Optional · must match {digits}:{value} pattern if set · max 100 chars
client_einvoice_sdi_code	Optional · exactly 7 alphanumeric chars OR the literal 0000000 if set
client_einvoice_pec_email	Optional · valid email format if set · max 100 chars
client_company_number	Optional · freetext · max 50 chars · no format enforcement

Acceptance criteria

• E-Invoicing section visible on both client create and client edit forms
• All four fields present with correct labels, placeholders, and tooltip / help text
• Validation errors shown inline without losing other form field values
• Section collapses / is hidden when all four fields are empty
• Company registration number field positioned near existing client identification fields