Fix unusable personal settings form: v8 Vue idioms and Deck relations

The settings page rendered, but nothing in it worked:

- All NcSelect dropdowns showed "undefined" for every option. In
  @nextcloud/vue 9 the `label` prop is vue-select's option display *key*,
  not a caption, so `label="Board"` read `option.Board`. Use `input-label`.
- Saving always failed with "Bitte einen Titel angeben". Vue 3 dropped
  `.sync`; NcTextField and NcCheckboxRadioSwitch bind via `modelValue`, so
  `:value.sync` / `:checked.sync` never wrote back. Use `v-model`.
- NcButton's style prop is now `variant`, `type` is the native button type
  and `native-type` is gone. `type="tertiary"` rendered
  `<button type="tertiary">`, which HTML falls back to `submit` for, making
  the cancel button submit the form.
- The user dropdown was always empty. Deck's RelationalEntity replaces
  resolved relations with a RelationalObject once an entity is enriched, so
  `$acl->getParticipant()` returns that wrapper and the uid lives in
  `getPrimaryKey()` -- probing for `getUID()` yielded null. This also broke
  the background job's assigned-user filter, which shares the extractor.
- A board's ACL never contains its owner (a private board has an empty
  ACL), so participants are now seeded with the owner, group ACL entries
  are expanded via IGroupManager and display names resolved via
  IUserManager.
- One board appeared twice: getUserBoards() merges own/group/circle boards
  and includes archived and trashed ones. Deduplicate by id and drop those.

Also keep one failing lookup in onBoardChange from taking the other two
dropdowns down with it, and document all of the above in CLAUDE.md.

src/PersonalSettings.vue

@@ -39,10 +39,10 @@
 						<td>{{ workflow.notifyEmail ? 'Ja' : 'Nein' }}</td>
 						<td>{{ workflow.enabled ? 'Ja' : 'Nein' }}</td>
 						<td class="wfda-actions">
-							<NcButton type="tertiary" @click="editWorkflow(workflow)">
+							<NcButton variant="tertiary" @click="editWorkflow(workflow)">
 								Bearbeiten
 							</NcButton>
-							<NcButton type="tertiary" @click="removeWorkflow(workflow)">
+							<NcButton variant="tertiary" @click="removeWorkflow(workflow)">
 								Löschen
 							</NcButton>
 						</td>
@@ -53,19 +53,19 @@
 				Noch keine Workflows angelegt.
 			</p>
 
-			<NcButton v-if="!showForm" type="primary" class="wfda-add-button" @click="startCreate">
+			<NcButton v-if="!showForm" variant="primary" class="wfda-add-button" @click="startCreate">
 				Workflow hinzufügen
 			</NcButton>
 
 			<form v-if="showForm" class="wfda-form" @submit.prevent="save">
 				<NcTextField
-					:value.sync="form.title"
+					v-model="form.title"
 					label="Titel"
 					required />
 
 				<NcSelect
 					v-model="form.boardId"
-					label="Board"
+					input-label="Board"
 					:options="boardOptions"
 					:reduce="option => option.value"
 					placeholder="Board wählen"
@@ -73,7 +73,7 @@
 
 				<NcSelect
 					v-model="form.sourceStackId"
-					label="Quell-Stapel"
+					input-label="Quell-Stapel"
 					:options="stackOptions"
 					:reduce="option => option.value"
 					:disabled="!form.boardId"
@@ -81,7 +81,7 @@
 
 				<NcSelect
 					v-model="form.targetStackId"
-					label="Ziel-Stapel"
+					input-label="Ziel-Stapel"
 					:options="stackOptions"
 					:reduce="option => option.value"
 					:disabled="!form.boardId"
@@ -89,7 +89,7 @@
 
 				<NcSelect
 					v-model="form.filterUserIds"
-					label="Nur für zugewiesene Benutzer (optional)"
+					input-label="Nur für zugewiesene Benutzer (optional)"
 					:options="participantOptions"
 					:reduce="option => option.value"
 					:disabled="!form.boardId"
@@ -98,18 +98,18 @@
 
 				<NcSelect
 					v-model="form.filterLabelIds"
-					label="Nur mit Label (optional)"
+					input-label="Nur mit Label (optional)"
 					:options="labelOptions"
 					:reduce="option => option.value"
 					:disabled="!form.boardId"
 					multiple
 					placeholder="Alle Labels" />
 
-				<NcCheckboxRadioSwitch :checked.sync="form.notifyEmail">
+				<NcCheckboxRadioSwitch v-model="form.notifyEmail">
 					Benachrichtigungs-E-Mail an mich senden
 				</NcCheckboxRadioSwitch>
 
-				<NcCheckboxRadioSwitch :checked.sync="form.enabled">
+				<NcCheckboxRadioSwitch v-model="form.enabled">
 					Workflow aktiv
 				</NcCheckboxRadioSwitch>
 
@@ -118,10 +118,10 @@
 				</NcNoteCard>
 
 				<div class="wfda-form-actions">
-					<NcButton type="primary" native-type="submit" :disabled="saving">
+					<NcButton variant="primary" type="submit" :disabled="saving">
 						Speichern
 					</NcButton>
-					<NcButton type="tertiary" :disabled="saving" @click="cancelForm">
+					<NcButton variant="tertiary" type="button" :disabled="saving" @click="cancelForm">
 						Abbrechen
 					</NcButton>
 				</div>
@@ -230,10 +230,11 @@ async function onBoardChange(boardId) {
 		return
 	}
 
+	// One failing endpoint must not take the other two dropdowns down with it.
 	const [stacks, labels, participants] = await Promise.all([
-		fetchStacks(boardId),
-		fetchLabels(boardId),
-		fetchParticipants(boardId),
+		fetchStacks(boardId).catch(() => []),
+		fetchLabels(boardId).catch(() => []),
+		fetchParticipants(boardId).catch(() => []),
 	])
 
 	stacksByBoard[boardId] = stacks