refactor(vis): H4.5 step 3 — collections endpoint and the SSR helper on the canonical resolver

collections.userVisibleLibraryIDs keeps its identity adapter and
delegates; the router's visibleLibraryIDsFor likewise (its identity
source is the session user_id string, the site-specific part it keeps).
The SSR site made the unification slog-proof: resolveLibrary — the SSR
one-of-a-kind (active selection + switcher rows + offline detection) —
stays untouched per the semantic diff.

internal/router/helpers.go

@@ -255,18 +255,5 @@ func visibleLibraryIDsFor(c *echo.Context, cfg *Config) (map[string]bool, []pgty
 	if err != nil {
 		return nil, nil, echo.NewHTTPError(http.StatusUnauthorized, "invalid user")
 	}
-	libraries, err := cfg.Queries.GetUserVisibleLibraries(c.Request().Context(), uuidToPGType(userU))
-	if err != nil {
-		return nil, nil, err
-	}
-	set := make(map[string]bool, len(libraries))
-	ids := make([]pgtype.UUID, 0, len(libraries))
-	for _, lib := range libraries {
-		if !lib.ID.Valid {
-			continue
-		}
-		set[lib.ID.String()] = true
-		ids = append(ids, lib.ID)
-	}
-	return set, ids, nil
+	return cfg.LibraryService.VisibleLibrarySet(c.Request().Context(), uuidToPGType(userU))
 }