feat(mui): fixed-width DataTable columns + restore Projects next-number hint

DataTable: optional per-column width; when any column sets one, the table uses table-layout:fixed with a colgroup so columns keep a stable size instead of reflowing as rows are filtered (overflow clipped with ellipsis). Tables without widths keep auto layout. Projects: assign column widths; restore the original '(přiděleno automaticky)' suffix on the next-number hint.

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

src/admin/pages/Projects.tsx

@@ -245,6 +245,7 @@ export default function Projects() {
     {
       key: "project_number",
       header: "Číslo",
+      width: "12%",
       sortKey: "project_number",
       mono: true,
       render: (p) => p.project_number,
@@ -252,6 +253,7 @@ export default function Projects() {
     {
       key: "name",
       header: "Název",
+      width: "16%",
       sortKey: "name",
       bold: true,
       render: (p) => p.name || "—",
@@ -259,16 +261,19 @@ export default function Projects() {
     {
       key: "customer",
       header: "Zákazník",
+      width: "13%",
       render: (p) => p.customer_name || "—",
     },
     {
       key: "responsible",
       header: "Zodpovědná osoba",
+      width: "14%",
       render: (p) => p.responsible_user_name || "—",
     },
     {
       key: "status",
       header: "Stav",
+      width: "10%",
       sortKey: "status",
       render: (p) => (
         <StatusChip
@@ -280,6 +285,7 @@ export default function Projects() {
     {
       key: "order",
       header: "Objednávka",
+      width: "12%",
       render: (p) =>
         p.order_id ? (
           <Box
@@ -300,18 +306,21 @@ export default function Projects() {
     {
       key: "start_date",
       header: "Začátek",
+      width: "8%",
       mono: true,
       render: (p) => formatDate(p.start_date),
     },
     {
       key: "end_date",
       header: "Konec",
+      width: "8%",
       mono: true,
       render: (p) => formatDate(p.end_date),
     },
     {
       key: "actions",
       header: "Akce",
+      width: "7%",
       align: "right",
       render: (p) => (
         <Box sx={{ display: "flex", gap: 0.5, justifyContent: "flex-end" }}>
@@ -443,7 +452,7 @@ export default function Projects() {
           nextNumberQuery.data?.number ??
           nextNumberQuery.data?.next_number ??
           "…"
-        }`}
+        } (přiděleno automaticky)`}
         submitText="Vytvořit"
         loading={creating}
       >

src/admin/ui/DataTable.tsx

@@ -17,6 +17,13 @@ export interface DataColumn<T> {
   bold?: boolean;
   /** If set, the header becomes a clickable sort control (requires onSort). */
   sortKey?: string;
+  /**
+   * Fixed column width (number = px, string = any CSS width incl. "%"). When
+   * ANY column sets a width the table switches to `table-layout: fixed`, so
+   * columns keep a stable size regardless of cell content (e.g. while
+   * filtering). Overflowing text is clipped with an ellipsis.
+   */
+  width?: number | string;
   render: (row: T) => ReactNode;
 }
 
@@ -47,9 +54,28 @@ export default function DataTable<T>({
     console.warn("DataTable: columns with sortKey require an onSort prop.");
   }
   if (rows.length === 0 && empty) return <>{empty}</>;
+  // When any column declares a width, lock the table to fixed layout so column
+  // widths come from the colgroup (stable) instead of from cell content (which
+  // jumps as rows are filtered). Tables that set no widths keep auto layout.
+  const hasWidths = columns.some((c) => c.width != null);
+  const colWidth = (w: number | string | undefined) =>
+    w == null ? undefined : typeof w === "number" ? `${w}px` : w;
   return (
-    <TableContainer sx={{ borderRadius: 2 }}>
-      <Table size="small" sx={{ "& td, & th": { borderColor: "divider" } }}>
+    <TableContainer sx={{ borderRadius: 2, overflowX: "auto" }}>
+      <Table
+        size="small"
+        sx={{
+          ...(hasWidths ? { tableLayout: "fixed", minWidth: 720 } : {}),
+          "& td, & th": { borderColor: "divider" },
+        }}
+      >
+        {hasWidths && (
+          <colgroup>
+            {columns.map((c) => (
+              <col key={c.key} style={{ width: colWidth(c.width) }} />
+            ))}
+          </colgroup>
+        )}
         <TableHead>
           <TableRow>
             {columns.map((c) => (
@@ -101,6 +127,13 @@ export default function DataTable<T>({
                     ...(c.mono
                       ? { fontFamily: "'DM Mono', Menlo, monospace" }
                       : {}),
+                    ...(hasWidths
+                      ? {
+                          whiteSpace: "nowrap",
+                          overflow: "hidden",
+                          textOverflow: "ellipsis",
+                        }
+                      : {}),
                   }}
                 >
                   {c.render(row)}