fix(imap): error instead of silent success when STORE/COPY/MOVE target UIDs aren't present

[kochj23]

Summary

DeleteMessages, CopyMessages, and MoveMessages in
internal/imap/client.go reported success even when the target UIDs were not
present in the selected mailbox. Removing a label an email didn't have (a COPY
to the wrong Labels/* folder, or a STORE on an absent UID) printed e.g.
Label removed from 1 message(s) while nothing actually changed.

Root cause, as described in the issue:

• STORE against absent UIDs is a valid no-op per RFC 3501 and returns no
  error; the streamed FETCH responses (one per modified message) were never
  counted.
• COPY of a UID set that matches nothing is accepted without error by many
  servers (Proton Bridge included); the CopyData (SourceUIDs/DestUIDs)
  was discarded.
• MoveMessages reimplements COPY + STORE inline and ignored both.

Fix

• DeleteMessages: drain storeCmd.Next() and error when the affected count
  is zero.
• CopyMessages: error when CopyData.SourceUIDs and DestUIDs are both
  empty.
• MoveMessages: apply both checks before expunging the source.

No extra IMAP round-trips — the affected count and COPYUID data are already in
the response streams. SetFlagsMultiple is intentionally left unchanged (per
the issue): a server returns zero affected items both for an absent UID and for
a flag that is already set, so distinguishing them needs a different approach.

Tests

Added internal/imap/client_affected_test.go, which drives a real in-memory
IMAP server (go-imap's imapmemserver, no new module dependency) end-to-end,
organized into all seven categories:

• Security – a failed (no-match) delete must not partially mutate the
  mailbox.
• Performance – present-UID delete completes promptly, documenting that the
  affected count adds no extra/blocking round-trip.
• Retry – a no-match delete keeps failing deterministically, never leaking
  a false success.
• Unit – missing UID → error; present UID → success (STORE affected count).
• Integration – MOVE actually relocates the message from INBOX to Archive.
• Functional – COPY and MOVE on a missing UID surface an error rather than
  silent success (the memserver rejects an empty COPY at the protocol level;
  Proton Bridge returns an empty COPYUID set — both paths are handled).
• Frame – N/A (no new wire framing; response parsing covered by the
  integration path); explicit skip placeholder retained.

go build ./... && go vet ./... && go test ./... all green.

Closes #11

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