Add fetchInternalDate; fix RFC 3501 zone rendering in date-time output

[jappeace-sloth]

Motivation

We migrate small webshops between hosting providers, and when a shop leaves its old platform the bundled mailbox is terminated with the subscription. To preserve the owner's mail we copy every message from the old account into the new provider's mailbox over IMAP — admittedly a slightly unusual use of an IMAP client library, but one we actually ran into. appendFull already lets us write a message with its original flags and an explicit date, but nothing in the library could read a message's INTERNALDATE, so the copied mail lost all its dates. We ended up hand-rolling a minimal client for this one gap; this PR upstreams the missing piece so we can use HaskellNet instead.

What's added

• fetchInternalDate :: IMAPConnection -> UID -> IO (Either FetchInternalDateError CalendarTime) — reads a message's INTERNALDATE, built on fetchByByteString like the other fetchers. Failures (InternalDateAbsent, InternalDateInvalid) are returned, not thrown, so the caller decides what a missing or malformed date means.
• stringToDatetimeIMAP :: String -> Either DatetimeParseError CalendarTime — the inverse of datetimeToStringIMAP; accepts the quoted wire form and space-padded single-digit days per the RFC 3501 grammar. DatetimeParseError has a constructor per failure mode, each carrying the offending fragment. Both functions and both error types are exported, so fetchInternalDate's result can be fed straight back into appendFull.

What's fixed

Adding the inverse exposed a bug in datetimeToStringIMAP's zone rendering: the offset-in-seconds was divided by 3600 and zero-padded to four digits, so UTC+1 was emitted as +0001 (i.e. one minute) and negative offsets produced garbage like -000-1. Any appendFull with a non-UTC CalendarTime sent a malformed or wrong date-time. The zone now renders as (+/-)HHMM per RFC 3501; show4 had no remaining users and is removed.

Tests

HUnit cases for rendering (including a negative half-hour zone), parsing, the render/parse round-trip, space-padded days, and rejection of malformed input, plus a scripted-connection test for fetchInternalDate in the style of the existing fetch API tests. All 64 cases pass (cabal test, GHC 9.10.3).