feat(odin): multi-invoice review scrolls; drag & drop attachments

- Review panel: flex children no longer shrink (flexShrink 0) — with 2+
  extracted invoices the cards kept compressing to fit 35vh, squashing the
  field grids with nothing to scroll; panel now scrolls, maxHeight 40vh.
- Drag & drop onto the chat column: depth-counted dragenter/leave (no
  flicker), Files-only, refused while busy, PDF/image filter with a Czech
  hint toast when nothing usable was dropped, channel-alpha overlay with
  dashed primary border ('Přetáhněte soubory sem'), staged through the same
  path as the paperclip.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/admin/components/odin/OdinChat.tsx

@@ -103,6 +103,10 @@ export default function OdinChat() {
   const fileRef = useRef<HTMLInputElement>(null);
   const threadRef = useRef<HTMLDivElement>(null);
   const seededId = useRef<number | null>(null);
+  // Drag & drop over the chat column. The depth counter pairs enter/leave
+  // events bubbling from children so the overlay doesn't flicker mid-drag.
+  const [dragOver, setDragOver] = useState(false);
+  const dragDepth = useRef(0);
 
   // Below md the conversation list collapses into a slide-in drawer.
   const isMobile = useMediaQuery((t) => t.breakpoints.down("md"), {
@@ -177,17 +181,63 @@ export default function OdinChat() {
   };
 
   // Attaching only stages files; nothing is sent until Odeslat.
-  const onFiles = (files: FileList | null) => {
+  const stageFiles = (files: File[]) => {
-    if (!files || files.length === 0) return;
+    if (files.length === 0) return;
     setAttachments((a) => [
       ...a,
-      ...Array.from(files).map((file) => ({ id: nextUid(), file })),
+      ...files.map((file) => ({ id: nextUid(), file })),
     ]);
+  };
+  const onFiles = (files: FileList | null) => {
+    if (!files || files.length === 0) return;
+    stageFiles(Array.from(files));
     if (fileRef.current) fileRef.current.value = "";
   };
   const removeAttachment = (id: string) =>
     setAttachments((a) => a.filter((s) => s.id !== id));
 
+  // Drag & drop anywhere over the chat column stages files exactly like the
+  // paperclip (same accept filter as the file input). Only file drags react —
+  // text/link drags pass through untouched — and nothing stages while busy.
+  const isAccepted = (f: File) =>
+    f.type === "application/pdf" || f.type.startsWith("image/");
+  const dragHasFiles = (e: React.DragEvent) =>
+    Array.from(e.dataTransfer.types).includes("Files");
+
+  const onDragEnter = (e: React.DragEvent) => {
+    if (busy || !dragHasFiles(e)) return;
+    e.preventDefault();
+    dragDepth.current += 1;
+    setDragOver(true);
+  };
+  const onDragOver = (e: React.DragEvent) => {
+    if (!dragHasFiles(e)) return;
+    // preventDefault permits the drop (and keeps the browser from replacing
+    // the app with the dropped file); while busy the drop itself is refused.
+    e.preventDefault();
+    if (busy) e.dataTransfer.dropEffect = "none";
+  };
+  const onDragLeave = () => {
+    if (dragDepth.current === 0) return;
+    dragDepth.current -= 1;
+    if (dragDepth.current === 0) setDragOver(false);
+  };
+  const onDrop = (e: React.DragEvent) => {
+    if (!dragHasFiles(e)) return;
+    e.preventDefault();
+    dragDepth.current = 0;
+    setDragOver(false);
+    if (busy) return;
+    const dropped = Array.from(e.dataTransfer.files);
+    const accepted = dropped.filter(isAccepted);
+    if (accepted.length === 0) {
+      // Nothing usable — hint instead of silently doing nothing.
+      if (dropped.length > 0) alert.info("Podporované soubory: PDF a obrázky");
+      return;
+    }
+    stageFiles(accepted);
+  };
+
   const onSelect = (id: number) => {
     setSidebarOpen(false);
     if (id === activeId || busy) return;
@@ -467,9 +517,14 @@ export default function OdinChat() {
         sidebar
       )}
 
-      {/* Chat column */}
+      {/* Chat column — also the drop target for invoice files (overlay below). */}
       <Box
+        onDragEnter={onDragEnter}
+        onDragOver={onDragOver}
+        onDragLeave={onDragLeave}
+        onDrop={onDrop}
         sx={{
+          position: "relative",
           flex: 1,
           minWidth: 0,
           display: "flex",
@@ -557,11 +612,14 @@ export default function OdinChat() {
         {review.length > 0 && (
           <Box
             sx={{
-              maxHeight: "35vh",
+              maxHeight: "40vh",
               overflowY: "auto",
               display: "flex",
               flexDirection: "column",
               gap: 1,
+              // Flex children shrink by default — with 2+ cards they'd get
+              // compressed to fit instead of overflowing into the scrollbar.
+              "& > *": { flexShrink: 0 },
             }}
           >
             {review.map((inv) => (
@@ -591,6 +649,63 @@ export default function OdinChat() {
           onRemoveAttachment={removeAttachment}
           onSubmit={submit}
         />
+
+        {/* Drop-target overlay — pointer-events pass through so the drag
+            events keep firing on the column beneath it. */}
+        {dragOver && (
+          <Box
+            sx={(t) => ({
+              position: "absolute",
+              inset: 0,
+              zIndex: 10,
+              pointerEvents: "none",
+              display: "flex",
+              alignItems: "center",
+              justifyContent: "center",
+              // Longhand on purpose — see the container border note above.
+              borderStyle: "dashed",
+              borderWidth: 2,
+              borderColor: "primary.main",
+              borderRadius: { xs: 0, md: 3 },
+              bgcolor: `rgba(${t.vars!.palette.primary.mainChannel} / 0.08)`,
+            })}
+          >
+            <Box
+              sx={{
+                display: "flex",
+                flexDirection: "column",
+                alignItems: "center",
+                gap: 0.5,
+                px: 3,
+                py: 2,
+                borderRadius: 2,
+                bgcolor: "background.paper",
+                boxShadow: 3,
+              }}
+            >
+              <Box
+                component="svg"
+                viewBox="0 0 24 24"
+                sx={{ width: 32, height: 32, color: "primary.main", mb: 0.5 }}
+                fill="none"
+                stroke="currentColor"
+                strokeWidth="2"
+                strokeLinecap="round"
+                strokeLinejoin="round"
+              >
+                <path d="M21 15v4a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2v-4" />
+                <polyline points="17 8 12 3 7 8" />
+                <line x1="12" y1="3" x2="12" y2="15" />
+              </Box>
+              <Typography variant="subtitle1" sx={{ fontWeight: 600 }}>
+                Přetáhněte soubory sem
+              </Typography>
+              <Typography variant="caption" color="text.secondary">
+                PDF nebo obrázky faktur
+              </Typography>
+            </Box>
+          </Box>
+        )}
       </Box>
     </Box>
   );