feat(ui): add Continue/Read button to series detail page

Add a prominent button next to Download on the series detail page that
navigates directly to the next book to read, removing the need to scroll
through the book list to find it.

The button picks the next book by sorting by number and selecting the
first in-progress book, or the first unread book if none are in progress.
It shows "Continue" for books with existing progress and "Read" for fresh
starts. Hidden when all books are completed. The Download button becomes
secondary (light variant) when the Continue button is present.

Reuses the same query cache as SeriesBookList so no extra API calls.

Author: AshDevFr

web/src/pages/SeriesDetail.tsx

@@ -20,6 +20,7 @@ import { useDisclosure } from "@mantine/hooks";
 import { notifications } from "@mantine/notifications";
 import {
   IconAnalyze,
+  IconBook,
   IconBookOff,
   IconCheck,
   IconChevronDown,
@@ -37,7 +38,7 @@ import {
   IconWand,
 } from "@tabler/icons-react";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
-import { useState } from "react";
+import { useMemo, useState } from "react";
 import { Link, useNavigate, useParams } from "react-router-dom";
 import {
   type PluginActionDto,
@@ -156,6 +157,29 @@ export function SeriesDetail() {
     enabled: canEditSeries && !!series, // Only fetch if user can edit series and series is loaded
   });
 
+  // Fetch books for this series to determine the next book to continue reading
+  const { data: seriesBooks } = useQuery({
+    queryKey: ["series-books", seriesId, false],
+    queryFn: () => seriesApi.getBooks(seriesId!),
+    enabled: !!seriesId,
+  });
+
+  // Find the next book to read: first in-progress book (by number), or first unread book
+  const nextBook = useMemo(() => {
+    if (!seriesBooks?.length) return null;
+    const sorted = [...seriesBooks].sort(
+      (a, b) => (a.number ?? 0) - (b.number ?? 0),
+    );
+    // Prefer the first book that is in-progress (has progress but not completed)
+    const inProgress = sorted.find(
+      (b) => b.readProgress && !b.readProgress.completed,
+    );
+    if (inProgress) return inProgress;
+    // Otherwise, the first book with no progress at all
+    const unread = sorted.find((b) => !b.readProgress);
+    return unread ?? null;
+  }, [seriesBooks]);
+
   // Mutation to fetch preprocessed search title before opening modal
   const searchTitleMutation = useMutation({
     mutationFn: (pluginId: string) => {
@@ -752,9 +776,26 @@ export function SeriesDetail() {
 
               {/* Action buttons */}
               <Group gap="sm" mt="xs">
+                {nextBook && (
+                  <Button
+                    size="xs"
+                    variant="filled"
+                    leftSection={<IconBook size={14} />}
+                    onClick={() => {
+                      if (nextBook.fileFormat === "epub") {
+                        navigate(`/reader/${nextBook.id}`);
+                      } else {
+                        const page = nextBook.readProgress?.currentPage ?? 1;
+                        navigate(`/reader/${nextBook.id}?page=${page}`);
+                      }
+                    }}
+                  >
+                    {nextBook.readProgress ? "Continue" : "Read"}
+                  </Button>
+                )}
                 <Button
                   size="xs"
-                  variant="filled"
+                  variant={nextBook ? "light" : "filled"}
                   component="a"
                   href={`/api/v1/series/${series.id}/download`}
                   leftSection={<IconDownload size={14} />}