feat(clients): optional billing email for invoicing + copy/toggle password buttons

[hedon77]

Adds an optional per-client billing (accounting) email address, separate from the sign-in address.

• New \clients.billing_email\ column + \Client::billingEmail()\ helper (falls back to the account email).
• Editable from admin client create/edit and the client profile.
• Invoice and payment-reminder emails now deliver to \�illingEmail()\ (manual send, bulk, events, cron).
• Shown in the client information panel.
• Adds copy + show/hide buttons to the admin password generator (matching the client area).

[Panelica]

Thorough — you found every billing send site. Please double-check that common.copy / common.toggle exist as translation keys (the button titles reference them). Otherwise good; we'll rebase this after #14.

[hedon77]

Confirmed — common.copy and common.toggle both exist in lang/en/common.php and lang/pl/common.php.

[Panelica]

Thank you — this was thorough; you found every billing send-site. We just merged the first clean batch into main: #16 (client delete), #24 (invoice number display), #13 (Tpay), #22 (addon settings framework) and #14 (tax rewrite). #14 (tax) and #16 (client delete) overlap this on the invoice and client files, so it now needs a rebase on the updated main. Once it's green again we'll merge. Much appreciated.

[hedon77]

*Zrebasowane na aktualny \main* — gałąź merguje się teraz czysto, konflikt zniesiony. Gotowe do merga.

[Panelica]

Thank you — this is good work, and it's safe by design (billing_email is optional and falls back to the account address). We just merged #9 and #21, which overlap this on the invoice and client files, so GitHub now shows a conflict. A rebase on the updated main and we'll merge it. Appreciated.

[hedon77]

*Zrebasowane na aktualny \main* (po zmergowaniu #9/#21) — konflikt zniesiony, gałąź merguje się czysto. Gotowe do merga.