Let each workflow watch the due date or the start date

Deck 1.18 added a start date to cards (Card::$startdate), so a workflow
no longer has to mean "overdue". Each rule now carries a date_field
('due' | 'start'), chosen in the settings form next to board and stacks.

Existing rules keep the due date: the new column's default supplies it
for every stored row, so there is no backfill step, and
Workflow::getDateFieldOrDue() covers entities that were never near the
database as well as hand-edited values. The controller is the one place
that rejects an unknown value instead of normalising it - a client
asking for a rule it would not get should hear about it.

The probe for the field is property_exists(), not method_exists():
Deck's entities declare no getter as real code - getStartdate(),
getDuedate(), even getId() all go through Entity::__call(), which
method_exists() ignores by definition. A method_exists() guard would
have been false on every Deck version and would have turned every
start-date rule into a silent no-op.

assertDeckAvailable() now also refuses Deck older than 1.18.0. That
floor cannot live in appinfo/info.xml - neither the server's nor the app
store's schema has an app-to-app dependency element - but
<nextcloud min-version="34"> already implies it, since 1.18.x is the
only Deck release line published for Nextcloud 34.

isOverdue() becomes hasPassed(): one function for both dates, since the
comparison and the argument against Deck's day-truncating
getDaysUntilDue() are identical for either.

Also carries a pending NcSelect icon-alignment fix that was already in
the working tree.

Bump to 34.1.0.

lib/Service/NotificationMailer.php

@@ -55,10 +55,19 @@ class NotificationMailer {
 			$template->setSubject($this->l10n->t('Deck card moved: %s', [$card->getTitle()]));
 			$template->addHeader();
 			$template->addHeading($this->l10n->t('A card was moved automatically'), false);
-			$template->addBodyText($this->l10n->t(
-				'The card "%1$s" was moved because it is overdue (workflow "%2$s").',
-				[$card->getTitle(), $workflow->getTitle()],
-			));
+			// Two whole strings rather than one sentence with an interpolated
+			// reason: the trigger is the point of the mail, and a translator
+			// needs to see it in the sentence to get the grammar right.
+			$reason = $workflow->getDateFieldOrDue() === Workflow::DATE_FIELD_START
+				? $this->l10n->t(
+					'The card "%1$s" was moved because its start date has been reached (workflow "%2$s").',
+					[$card->getTitle(), $workflow->getTitle()],
+				)
+				: $this->l10n->t(
+					'The card "%1$s" was moved because it is overdue (workflow "%2$s").',
+					[$card->getTitle(), $workflow->getTitle()],
+				);
+			$template->addBodyText($reason);
 			// Single-argument addBodyText() runs htmlspecialchars() itself, so
 			// these board- and stack-titles (user input) are safe as-is —
 			// unlike the description below, which passes both parts.