feat(sync): unlinked-books queue — ingest, page route, dashboard banner

The identification queue for device files the sync cannot match is now
live end-to-end. Previously the whole feature was dark: unmatched device
files were silently dropped (losing their reading progress), the queue
table sat empty by construction, and the page template had no route.

- Ingest: the KOReader sync loop, on an unresolvable file, upserts
  unlinked_books (one row per device + content id; re-syncs bump
  last_seen_at and refresh device-reported metadata) instead of
  dropping the file. Schema gains the (device_id, content_id) unique
  index the upsert needs.
- Page: GET /unlinked-books routes the existing page template, listing
  the caller's own unresolved rows (ListUnresolvedUnlinkedBooksForUser
  — devices.user_id scoping; no cross-user leakage).
- Dashboard banner: a warning card above the shelves, rendered only
  when the caller's queue is non-empty (hidden-unless-needed), linking
  to the page.
- Resolve flow unchanged and already gated: MatchingHandler.LinkBook
  verifies device ownership and target-item visibility (403s) and
  creates the device file alias, so the next sync matches by path.
  SyncHandler.LinkBook and Handler.LinkBook are unrouted dead handlers
  — triage candidates, not part of the live flow.

internal/router/frontend.go

@@ -475,8 +475,18 @@ func registerFrontendRoutes(cfg *Config) {
 			}
 		}
 
+		userUUID, userUUIDErr := uuid.Parse(user.ID)
+		if userUUIDErr != nil {
+			return renderErrorPage(c, "Error loading user", "user_load_error")
+		}
+		unlinkedCount, countErr := cfg.Queries.CountUnresolvedUnlinkedBooksForUser(c.Request().Context(), pgtype.UUID{Bytes: userUUID, Valid: true})
+		if countErr != nil {
+			log.Printf("CountUnresolvedUnlinkedBooksForUser failed: %v", countErr)
+			unlinkedCount = 0
+		}
+
 		var buf bytes.Buffer
-		err = templates.Dashboard(user, sectionData, allSectionsData, libRes.Libraries, libraryID, prefs.HiddenCollections, limit, week, activeLibType, errorMsg).Render(c.Request().Context(), &buf)
+		err = templates.Dashboard(user, sectionData, allSectionsData, libRes.Libraries, libraryID, prefs.HiddenCollections, limit, week, activeLibType, errorMsg, int(unlinkedCount)).Render(c.Request().Context(), &buf)
 		if err != nil {
 			return err
 		}
@@ -742,6 +752,45 @@ func registerFrontendRoutes(cfg *Config) {
 	// itself here (htmx) whenever the library switcher dispatches
 	// week-refresh, carrying the current #library-select value. Same
 	// resolution rules as the dashboard page (family-lens safe).
+	// Unlinked-books page: the identification queue for device files the
+	// sync could not match (personal data — the caller's own devices only).
+	frontendProtected.GET("/unlinked-books", func(c *echo.Context) error {
+		user, err := getTemplateUserWithTheme(c, cfg)
+		if err != nil {
+			return renderErrorPage(c, "Error loading user", "user_load_error")
+		}
+		userUUID, userUUIDErr := uuid.Parse(user.ID)
+		if userUUIDErr != nil {
+			return renderErrorPage(c, "Error loading user", "user_load_error")
+		}
+		rows, listErr := cfg.Queries.ListUnresolvedUnlinkedBooksForUser(c.Request().Context(), pgtype.UUID{Bytes: userUUID, Valid: true})
+		if listErr != nil {
+			log.Printf("ListUnresolvedUnlinkedBooksForUser failed: %v", listErr)
+			rows = []database.ListUnresolvedUnlinkedBooksForUserRow{}
+		}
+
+		books := make([]templates.UnlinkedBookData, 0, len(rows))
+		for _, r := range rows {
+			books = append(books, templates.UnlinkedBookData{
+				ProgressID:      uuid.UUID(r.ID.Bytes).String(),
+				DeviceID:        uuid.UUID(r.DeviceID.Bytes).String(),
+				DeviceType:      r.DeviceType,
+				DeviceName:      r.DeviceName,
+				FilePath:        utils.PgTextString(r.FilePath),
+				SHA256:          r.ContentID,
+				TitleFromDevice: utils.PgTextString(r.Title),
+				LastSyncTime:    utils.FormatTimeInTimezone(r.LastSeenAt.Time, user.Timezone),
+				ConfidenceScore: r.ConfidenceScore.Float64,
+			})
+		}
+
+		var buf bytes.Buffer
+		if renderErr := templates.UnlinkedBooks(user, books).Render(c.Request().Context(), &buf); renderErr != nil {
+			return renderErr
+		}
+		return c.HTML(http.StatusOK, buf.String())
+	})
+
 	// Unlinked-book match results partial: the match cascade for one device
 	// file, rendered server-side and scoped to the requester's visible
 	// libraries (absent-from-results) — rendering-ownership: templ renders,