feat(admin): manual session create/edit form

Add the shared SessionForm component (create/edit) used by the Sessies screen.
- Create mode: worker picker (useAdminUsers), activity (useActivities), zooltype,
  pair count, start/end datetime-local, pauze, notitie; posts CreateManualSessionInput.
- Edit mode: hides the worker picker, prefills from the row session, adds a status
  select; PUTs AdminUpdateSessionInput (end_time nulled when status=active).
- Builds ISO start_time/end_time from the datetime-local values; live "gewerkt"
  preview = end - start - paused; inline error on a 400.
- Wires the form into Sessions.tsx ("+ Nieuwe registratie" -> create, pencil -> edit).

Products affected: SoleLog admin client

apps/admin/src/styles.css

@@ -577,3 +577,40 @@ body {
   border-radius: 12px;
   cursor: pointer;
 }
+
+/* ---- Session create/edit form ---- */
+.session-form {
+  gap: 14px;
+}
+
+.session-field {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
+.session-field select,
+.session-field input,
+.session-field textarea {
+  padding: 10px 12px;
+  font-size: 15px;
+  border: 1px solid var(--border);
+  border-radius: 12px;
+  background: #ffffff;
+  color: var(--text);
+  font-family: inherit;
+}
+
+.session-field select:focus,
+.session-field input:focus,
+.session-field textarea:focus {
+  outline: none;
+  border-color: var(--primary);
+}
+
+.session-preview {
+  margin: 0;
+  font-size: 15px;
+  color: var(--text-muted);
+  font-variant-numeric: tabular-nums;
+}