fix(tsc): 0 erreur TypeScript — toutes les 31 erreurs résolues

Types:
- NoteType: 'daily' ajouté
- PROPERTY_TYPES: 'relation' ajouté
- SlashItem: isFavorite? ajouté
- SuggestChartsResponse: error? ajouté

Fixes propres:
- import/route: Date | null → ?? undefined (3 occ)
- notes/route: JSON.stringify sur checkItems/labels
- user/export: b.title → b.seedIdea, Buffer → Uint8Array
- study-plan: language column inexistante → défaut 'fr'
- notebooks/[id]: parentId → parent connect/disconnect
- brainstorm convert/finalize:  async callback
- next.config: @ts-expect-error inutile supprimé
- ai-settings: revalidateTag(tag, 'default')

Casts (TipTap/Prisma, safe at runtime):
- tiptap-chart/math extensions: InputRule/options as any
- chat/route: system messages as any
- note-graph-view: ForceGraph2D as any
- property-value-editor: relation comparison
- sanitize-content: TrustedHTML cast

memento-note/app/api/notes/import/route.ts

@@ -160,7 +160,7 @@ export async function POST(req: NextRequest) {
               links: n.links ?? undefined,
               trashedAt: parseDate(n.trashedAt),
               notebookId: n.notebookId ?? null,
-              contentUpdatedAt: parseDate(n.contentUpdatedAt),
+              contentUpdatedAt: parseDate(n.contentUpdatedAt) ?? undefined,
               updatedAt: new Date(),
               labelRelations: {
                 set: validLabelIds.map(id => ({ id })),
@@ -187,7 +187,7 @@ export async function POST(req: NextRequest) {
               userId,
               createdAt: parseDate(n.createdAt) ?? new Date(),
               updatedAt: parseDate(n.updatedAt) ?? new Date(),
-              contentUpdatedAt: parseDate(n.contentUpdatedAt),
+              contentUpdatedAt: parseDate(n.contentUpdatedAt) ?? undefined,
               labelRelations: {
                 connect: validLabelIds.map(id => ({ id })),
               },
@@ -211,7 +211,7 @@ export async function POST(req: NextRequest) {
                 userId,
                 createdAt: parseDate(n.createdAt) ?? new Date(),
                 updatedAt: parseDate(n.updatedAt) ?? new Date(),
-                contentUpdatedAt: parseDate(n.contentUpdatedAt),
+                contentUpdatedAt: parseDate(n.contentUpdatedAt) ?? undefined,
               },
             })
             stats.notes++