fix(plan): keep retired category usable when editing an entry's note

Update validates the category only when it changes, so a note-only edit on an entry whose category was later hidden/deleted no longer 400s. The edit select also shows the current (retired) category marked '(skryta)' instead of silently snapping to the first active option; create defaults to an active category.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/components/PlanCellModal.tsx

@@ -122,7 +122,11 @@ function EditForm(props: Props) {
         date_to: mode.date,
         is_range: false,
         project_id: null as number | null,
-        category: "work",
+        // Default to "work" when it's active, otherwise the first active
+        // category, so a new entry never starts on a retired key.
+        category: activeCategories.some((c) => c.key === "work")
+          ? "work"
+          : (activeCategories[0]?.key ?? "work"),
         note: "",
       };
     }
@@ -294,6 +298,19 @@ function EditForm(props: Props) {
             value={category}
             onChange={(e) => setCategory(e.target.value)}
           >
+            {/* If the row's current category was retired/hidden, still show it
+                (marked) so the select reflects reality instead of silently
+                snapping to the first active option. The server only re-checks
+                the category if it changes, so keeping it saves fine. */}
+            {category && !activeCategories.some((c) => c.key === category) && (
+              <option value={category}>
+                {planCategoryLabel(
+                  category,
+                  Object.fromEntries(props.categories.map((x) => [x.key, x])),
+                )}{" "}
+                (skrytá)
+              </option>
+            )}
             {activeCategories.map((c) => (
               <option key={c.key} value={c.key}>
                 {c.label}