🐛 Fixed Mastodon social account field mangling input while typing (#28255)

fixes https://github.com/TryGhost/Ghost/issues/27785

The Mastodon field in Settings → Social Accounts rewrote the input
while the user was still typing, making handles like
@username@instance.tld and federated URLs like
https://fedi.pub/@user@instance.com impossible to enter. The field
would reorder mid-typing — e.g. once "@user@instance.co" became a
valid FQDN, the value was canonicalised to "https://instance.co/@user"
before the user finished typing ".com", dropping characters and
jumping the cursor.

The cause was not the validation regex but when normalisation ran.
Every keystroke calls updateSetting, which changes the stored handle
and triggers the resync effect to overwrite the visible field with the
canonical display value. For platforms whose canonical form is a
prefix plus the raw tail (Instagram, etc.) the cursor stays at the end
and typing is unaffected, but the Mastodon handle form reorders tokens
(@user@instance → https://instance/@user), which corrupts in-progress
input.

The fix tracks which field is focused and leaves the user's raw text
untouched while they are editing it, canonicalising once on blur and
only when the value is valid — invalid input is left as typed with its
error rather than reverted. This matches the behaviour the existing
testUrlValidation helper already relied on (fill → blur → expect
canonical), so every platform keeps its canonicalised display value
once editing finishes.

Covered by acceptance tests that type a handle character-by-character
(asserting the field is not rewritten mid-typing, then canonicalises on
blur) and that a federated Mastodon URL can be entered without error.
One existing Instagram test was updated to blur before asserting the
canonical value, since it previously relied on canonicalise-while-
focused.