Refactor mail sending and improve invoice/payment admin UX

[nspo]

view_helpers:

• Add EmailConnectionWrapper: wraps Django mail connection to enable unification of mail sending with create_invoices later.
• Extract HTML mail generation as helper functions
• Refactor send_invoice_mails() and send_reminder_mails() to use the wrapper; eliminate nested try/except in favor of bool checks
• send_invoice_mails() now requires users_autolocked and rolls back autolocks on mail failure or connection error
• Extract create_invoices() from InvoiceCreateView; tracks and reports auto-unlocks
• send_reminder_mails() skips users with balance >= 0, reports skipped count
• Set context["subject"] in all mail-sending paths and debug views

views / forms:

• PaymentForm: restrict user dropdown to self-paying users; block others on submit
• InvoiceCreateView: pre-select user via ?user= query param (get_initial)

UI:

• Invoice creation: Show filter box for user names (kind of a hack)
• User detail: "Create invoice" button (self-paying users only), payment reminder button (balance < 0 only), mail preview footer buttons
• Payment detail: always show "Generate invoice for user" button
• Invoice detail: mail preview footer button

[nspo]

I think I'll commit a sin here and merge this before independent review... Need to have this go live quickly. I let Claude "review" it and checked the potential issues, which IMO are fine:

● Review of feature/quick_invoice

Four potential issues were identified and evaluated:

1. generate_email_invoice exception orphans invoice — Dismissed: template rendering failures are not a realistic concern with stable templates.
2. Dependant notification failures bleed into the invoice-abort threshold — Dismissed: intended behavior.
3. send_messages returning 0 silently treated as success — Dismissed: assumed fine in practice.
4. continue after invoice delete skips auto-unlock — Dismissed: correct behavior since no invoice was issued, so no state change should occur.

Conclusion: no changes needed, branch is good to merge.