app: remove duplicated and dead code

upgradeLegacy assembled every remote, folder and schedule twice: once as
Remote and Folder objects to validate, and again as a parallel JSON tree
to return. Build the objects and serialise them, and drop the dead
endpointAllowed branch, the redundant pin emptiness test, and the
excludes length pre-check that excludesAllowed already makes.

atomicWrite existed verbatim in Config and Keys and was open-coded a
third time in LastSync, which then reached into Config's companion for
the bounded reader. The same bounded read was open-coded twice more, for
the encrypted key and for an imported one. Both are plain file
operations on the same private directory and belong in one place, with
readText a thin wrapper over readBytes.

Generating and importing an identity key ran the same twenty lines of
worker thread, error capture, refresh, toast and dialog; only the action
and two strings differed. rotateIfBig rotated at length >= MAX_BYTES,
which line() already subsumes, and its only caller invoked it
immediately before a line(). notifyError and notifyDeleted were the same
builder twice. openSettings nested a try/catch and repeated one toast;
take the candidates as a vararg. RemoteActivity.save defaulted its
argument to current(), which validates and writes the status field, so
the default hid that side effect from one of two callers.

Dead on arrival: Outcome.deleted's default, which no caller omits; the
rsa-sha2-* arms of hostKeyFile, since rsh prints PublicKey.Type() and an
RSA host key is always ssh-rsa; and the limit <= 0 branch in
SyncLog.text, which tail already handles and handled more accurately.
The config recovery note is now written with an explicit charset like
every other write here.

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