Fixed: email subject lines display correctly

Two related problems affected the subject lines of Subi's notification emails. The default billing reminder's subject, "[Subi] Reminder of upcoming payment," said nothing about which store or when. Separately, any email subject was rendered through the same escaping used for HTML page content, so a store name with an ampersand, apostrophe, or similar character showed up in the subject line as literal HTML codes, for example "Tom & Jerry's" instead of "Tom & Jerry's."

The billing reminder subject now reads "Your {store} payment is coming up on {date}," naming the store and the date directly.

Every email subject across the app now renders as plain text on one line, covering contract notifications, portal emails, digests, dunning, and invoices, not just the billing reminder. A subject with no special characters to begin with is unaffected.

This was purely a display fix in the subject line; the content and formatting of the email body itself did not change.